There is a well known part of speech called "adverbs". However, many adverbs are not grammatically interchangeable with each other. From Wikipedia:
Adverbs are traditionally regarded as one of the parts of speech. However, modern linguists note that the term "adverb" has come to be used as a kind of "catch-all" category, used to classify words with various different types of syntactic behavior, not necessarily having much in common except that they do not fit into any of the other available categories (noun, adjective, preposition, etc.)
Adverbs as a "catch-all" category
- She gave birth naturally.
- Naturally, she gave birth. different meaning of "naturally"
- Perry is very fast.
- *Perry very won the race. only some adverbs can go in this position
- The sock looks good there.
- *It is a there beautiful sock. only some adverbs can go in this position
What can we call groups like this?
Adverbs are not always grammatically interchangeable because they are ____. This means we have to classify this word group into specific subgroups before they can be truly useful.
I'm looking for a generic term, not one specific to linguistic terminology. Noun, adjective, it doesn't matter. Feel free to completely reword the example sentence as long as the phrase is relatively simple. It would be good if the phrase could also be used as a label:
Heterogeneous has the meaning I want (not all the members are the same, but some can be; different from all members being different), but neither it nor any of its single word synonyms are plain enough for me:
I think it's unlikely a single word will get the meaning across in a way that most people would understand, so a phrase seems more likely. The best I can come up with so far is "uneven group":
Adverbs (uneven group)
Adverbs are not always grammatically interchangeable because they are an uneven group.