From grammar.300:
Absorb all members of selma'o DAhO, FUhO, FUhE, UI, Y, and CAI into the previous token. All of these null grammar tokens are permitted following any word of the grammar, without interfering with that word's grammatical function, or causing any effect on the grammatical interpretation of any other token in the text. Indicators at the beginning of text are explicitly handled by the grammar.
"...permitted following any word (...), without (...) causing any effect on the grammatical interpretation ...". This should be qualified. These indicators cannot be introduced following zoi, zo or zei, or between any word and zei, bu, nai or si, without having a grammatical effect. Following ba'e they also interfere somewhat.
They can't appear after fa'o.