FileMaker will sometimes let you store data in a field even though the data does not fit the field definition. The return delimited data as you have seen can be stored in a field that is defined as a number. However it will not correctly handle that data. Say for instance you have a record with the checkboxes 2, 3, 4 & 5 checked. It will consider that data as one number "2,345" ie. two thousand, three hundred and forty five. In an edit box it would display such data as if it was return delimited:
This data ought to be in a text field not a number field. Similarly, it is possible to enter numbers into text fields, but they will not sort correctly as they would if they were entered in number fields: About choosing a field type
It is up to the developer to set up the data entry mechanisms to ensure data integrity so that you only have number data in number fields and have text data in text fields.