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.
There isn’t really any guessing, so I suppose that wording is poor.
The result of the formula basically determines the type (number, text, date, checkbox, entity, collection,etc.) with a couple of ‘sub-options’ for some of them, e.g.
number - number, currency or percent
text - text, phone, email or URL
date - date or datetime
So for example, if your formula is Name = “Hello” then the result will be a checkbox, and that’s not something that the user can change.
Thanks for clarifying! Putting that in the docs would be 
I’ve updated our guide, so I hope it is now clearer.