What is the difference between the ‘omnifunc’ and ‘completefunc’ autocomplete in Vim, except for the invocation key mapping?

Question

Is there any difference in functionality between these two types of complete-functions in Vimscript?

Answer 1

Technically, there is no difference in implementation of both User-defined completion (see :help compl-function, :help 'completefunc') and Omni completion (see :help compl-omni, :help 'omnifunc'). Both of them work by calling a function of a special semantics that locates the start of the expression to be completed at the current position, and provides appropriate completion suggestions (see :help complete-functions).

Where the difference between these two types of completion manifests itself, is the purpose of completion. User-defined completion should be customized by the user themselves according to conditions specific to a particular completion idea. Omni completion is supposed to be filetype-specific. Hence, separate Omni completion functions for different filetypes are loaded automatically by Vim from autoload/‹ft›complete.vim files, where ‹ft› stands for a particular filetype. See those Omni completion files for C or Python filetypes as a reference.