I have a custom module with some utility (can be used for other modules) and internal (can be used only for the module) functions inside, so I decide to place this functions in another file.

But reading the coding standards I can't find a place for this, I have as a candidate src/Utility/ for the utility functions and then create some files related to the utility category like in core ([Utility classes and functions][1]). This will be made in a new module and this new module will be added as dependency, in this way I will have in only one place all the common utilities functions and the other modules (that can need the functions) can add this new 
module as a dependency too.

But for the internal functions I'm thinking to use /src/Utility/InternalFunctions.php as file for this.

So, where is the correct place for the utility and internal functions for a custom module?


  [1]: https://api.drupal.org/api/drupal/core!core.api.php/group/utility/8.2.x