There are a number of cases where layout algorithms in CSS (and perhaps other things) care about whether a value has a percentage. In particular, values that have a percentage are treated differently when there's nothing for the percentage to resolve against: inside of something with auto height a height: calc(30px) is a fixed value but a height: calc(30px + 0%) is treated as height: auto.

With calc() as it was specified in css-values-3, this could essentially be implemented as part of the type computation; a calc() expression could effectively be either a <length> or a <length-percentage> as its toplevel type, and those that were <length-percentage> act as though they have a percent.

Newer levels of the specification introduce features that make this approach insufficient:

• division by values with units
• sign()
• progress() and related functions
• calc-size()

(I think this is clearly defined for calc-size() because it's very important there, but I don't think it's clearly defined for the other cases.)

It should be clearer whether these other things "erase" the percentage-ness of their arguments when they erase the types of those arguments, or whether they still produce toplevel values that are treated as having a percentage.