Think of your tool tip as an unstored calculation field defined inside the same record as the field that you associate it with. (and if you use a tool tip on a button, think of it as an unstored calculation field in the same record as the layout record that is "current" should that button be clicked.
That's the context in which a tool tip calculation evaluates.
Does that then mean that if a tooltip is for:
1. A field, if I store tool tips in fields, those fields need to be part of the same record as the fields to which it is associated? ie. the tool tips would be stored on the same table?
2. A button, as 1. but it will be the current record?
If that's the case, it looks like it would be v. wasteful to store tool tips as part of a record.
You cannot store tool tips in fields. The context for how a tool tip evaluates is the same for any tool tip you define on a given layout whether button or field or ....
Tool tips are not stored as part of a record, they are part of the layout.