Here is the wording from Heidenhain’s website:
Air Supply for Linear Encoders
A tiny detail can make a big difference: the HEIDENHAIN connecting piece with built-in throttle plays a key role in the performance of linear encoders by ensuring optimal air pressure.
Uniform air supply
Why is this important? Encoders located within the machine envelope are supplied with sealing air through various lengths of tubing. As a result, different volumetric flow rates can arise when conventional connectors are used. In a short tube, flow rates are high, and the encoder receives too much sealing air. But in very long tubes, friction loss takes its toll, and the encoder receives too little sealing air.
The right connecting piece
Since the tube lengths are unalterably defined for the machine layout, the solution lies in the selection of the connecting piece. The HEIDENHAIN connecting piece with built-in throttle ensures optimal air pressure within the encoder (HEIDENHAIN recommends 7 l/min to 10 l/min). The throttle nozzle incorporated into the HEIDENHAIN connecting piece acts as a regulator, equalizing the various flow rates within the tubes.
Reduced loss of flow
This feature ensures a defined volumetric flow rate through the linear encoder, regardless of the specific tube topology. Loss of flow is reduced to a minimum, and air supply distribution becomes more uniform. The HEIDENHAIN connecting piece with built-in throttle thereby creates the conditions under which HEIDENHAIN linear encoders can attain their high level of performance.