How to name a function is described in Naming Conventions which says:
Functions should be named using lowercase, and words should be separated with an underscore. Functions should in addition have the grouping/module name as a prefix, to avoid name collisions between modules.
That is valid for hooks and other functions implemented from the module.
For functions that are not hooks, you could also prefix them with an underscore, right before the grouping/module name. If you do it, you are saying to the developers of other modules that the function is not supposed to be called from other modules, but it is private for the module defining it.