Because an empty station is an open port on a pressurised manifold - it does not just look untidy, it leaks continuously.

The manifold distributes supply air to every station along its length. A station with no valve fitted leaves that supply passage open to atmosphere at that position, so the terminal blows air constantly.

A blanking plate seals the station, using the same mounting interface and seals a valve would, so the manifold is closed and the position remains available.

That is the second reason they matter: a blanked station is a station that can later take a valve. The manifold, the tubing routes and the electrical addressing are already there, so adding a function is a matter of fitting a valve and connecting its working lines.

Building a terminal with a few blanked positions is therefore a deliberate provision for change, not a sign of over-specification.

By exhausting the terminal, removing the retaining screws and lifting the module off the manifold - the porting and the electrical connection are both made through the mounting interface.

Valve modules seat on the manifold face with seals around the port openings, and the electrical contact is made through the same interface or through a connector on the module. Two screws typically retain each one.

So changing a valve does not disturb any tubing except that station's own working lines, and disturbs no wiring at all.

The terminal must be isolated and exhausted first, because the manifold is a common supply - which means the whole block loses pressure, not just the station being worked on. Where that matters, some ranges offer per-station isolation.

Changing a valve for a different function - a five-port for a three-port, say - is equally straightforward mechanically, but it changes what that station does, so the electrical mapping and the machine documentation must be updated to match.

They divide the manifold's internal passages so that a group of stations can have its own supply pressure or its own exhaust.

By default the manifold runs one supply passage and one exhaust passage along its whole length, so every station sees the same pressure and shares the same exhaust route.

A supply separator blocks the supply passage at a chosen point and provides a new inlet, so the stations beyond it are fed independently. That allows part of the terminal to run at a lower pressure - a gentle clamping function among firm ones, or a low-pressure blow-off - without a second terminal.

An exhaust separator does the same for the exhaust, which matters where some valves must exhaust to a collected or piped-away outlet - in a cleanroom, or where the exhaust carries contamination - while others vent locally.

Both are specified at build time as part of the station schedule, and their positions should be recorded, because their effect is invisible from outside the block.

Enough to absorb foreseeable change, which in practice means a small proportion of the total rather than an exact fit.

The cost of a blanked station at build time is a blanking plate and a little manifold length. The cost of needing one later is a new manifold section or a separate valve mounted elsewhere, with new tubing and possibly re-addressing.

Machines change: an extra clamp, a reject mechanism, an additional gripper. Building the terminal to exactly today's requirement guarantees that the first modification is disproportionately expensive.

A common approach is to allow a couple of spare stations on a small terminal and a modest percentage on a larger one, positioned at the end where they are easiest to bring into use.

It is also worth checking that the electrical node has spare addresses to match - a spare pneumatic station with no available output is not much use.

No - modules are specific to the manifold system they were designed for, and interfaces differ between manufacturers and between ranges from the same manufacturer.

The mounting interface carries the porting, the seals and often the electrical contacts, all in a defined geometry. A module from another range will not seal, will not port correctly, and may not make electrical contact even where it appears to fit.

So when extending or repairing a terminal, the range and generation must be identified precisely. Manufacturers publish part numbers on the modules themselves, and recording the terminal's full specification in the machine documentation saves considerable time later.

This also matters for obsolescence. A terminal range that has been superseded may no longer have matching modules available, and extending it later becomes impossible - which is a further argument for specifying spare stations while the range is current.

They hold the assembly together and terminate the manifold's internal passages.

A modular terminal is a stack of manifold sections clamped between two end plates, usually by tie rods running the length of the block. The clamping force compresses the seals between sections, which is what makes the internal supply and exhaust passages airtight.

The end plates carry the main supply and exhaust connections and close off the passages at each end. On many ranges one end plate also carries the electrical node or connector.

The practical implications are that the tie rods must be tightened evenly and to the specified torque when a terminal is extended or rebuilt - uneven or insufficient clamping produces leaks between sections that are hard to locate - and that adding sections means longer tie rods, which have to be ordered with the extension.

Check the assembly instructions rather than assuming; the sequence and torque matter.

As a table mapping each physical position to its function, its valve type and its controller address - kept with the machine, not just in the design file.

A valve terminal presents a row of identical-looking modules. Without documentation, relating station seven to a machine function means tracing tubes.

The schedule should record, for each position: the station number, the valve function fitted, the machine function it performs, the controller output address, and whether it is blanked or spare. Supply and exhaust separator positions belong in it too, since their effect is not visible.

Label the stations physically as well, with the machine function rather than just a number - it is what a technician needs at two in the morning.

Update it whenever a module is changed. An out-of-date schedule is worse than none, because it is trusted.