It seems like Geany offers 2 ways of helping complete what you're in the middle of typing:
1. Autocomplete construct -- where you hit Tab and get completion of the preceding construct, which is defined in your autocomplete.conf file, and
2. Complete word -- where you hit Ctrl-Space and get completion of the preceding partial word, which is a symbol somewhere else in the file which Geany knows about via its built-in understanding of tags.
Is that correct?