The API 609 deifinition reads as,

"The term trim refers to shafts, seating surfaces on the body and disc, and internal keys, pins, screws and bushings in contact with the contained fluid are also considered trim".

Why is the disc not treated as a trim part? Why is it the seating surface alone of the disc is treated as the trim.

Remember the origin: American Petroleum....

I would imagine disc not described as trim derivates from the normal use of descriptions within the branch where body and disc are separately described (in detail) and with demands on material and other tests for the actual application, and that trim (as described above) might vary and also have a different set of demands.

Also the word in itself 'trim' describes in my opinion 'lesser details to finalize or make ready for use, and attached or in connection to an already existing or described larger item or construction' (My own interpretation-Filter)

Other opinions?

The intent of only stating "seating surfaces on the body and disc" is to avoid someone thinking that the entire part must be made from corrosion resistant material.  Sometimes coating or facing the sealing surfaces with corrosion resistant or harder material is adequate for the service.  And usually it costs less.

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