I was trying to find some guidance on formula types and eventually got here in the FIbery docs:
You will see Formula Field creation form. In this form specify the Field Name and the Formula itself. The returned data type will be guessed automatically, but in some cases you can configure it. For example, for Number you may select Decimal or Integer.
It would be helpful to see a table of formula types with: a description of each type and what heuristic is used to guess the type automatically.