Hi Adam
Once you add a custom field of a certain type it may not be possible to switch it to another type because of how the data are stored.
You can effectively swap between fields that are stored as simple text strings, such as the Single Line field itself and the email field, the number field, the phone field etc. Switching one of these to, say, a checkboxes field which is stored as a complex array would break for existing data, and hence is not permitted.
Some fields allow multiple instances. The email field for example, you can designate as a single or repeating field, in which case you can add multiple email addresses to a single post. More complex fields such as radio, individual checkbox or multiple checkboxes fields cannot have multiple instances of that field.
A single checkbox field may be used for an option such as "In Stock" which has a binary choice. A checkboxes fields would be used where multiple options could be selected, e.g. "Available Colours" with options for Red, Green, Gold etc.
Repeatable Field Groups are not to be confused with repeating individual fields (i.e. fields which permit multiple instances).
An example of how you might use Repeatable Field Groups would be for offices and contact details. You have offices in Milan, Paris, and London. Each office has a contact person name and an email address. You could make repeating fields for the Contact (a single line field) and email address, but you couldn't be sure which contact went with which email address, there would just be 3 of each.
So you make a repeatable field group containing both fields. You add one group, for London, where you add a singular contact and a singular email address. You add a second group, for Milan, where you add the contact and their email. Then again for Paris.
I hope that clarifies things for you a little.
The documentation page you were looking at previously was probably https://toolset.com/documentation/customizing-sites-using-php/functions/
