What are Custom 1/2/3 used for? Can I rename these fields?

These are the 3 default custom fields that you can import any value into. For example, you could use Custom 1 to import and store a value for Summer Born pupils.

These fields cannot be renamed, however, it is possible to create your own custom fields using Admin > Edit Pupils > Edit Custom Fields.

Data can be added to these fields manually using the relevant cell to the bottom right of Edit Pupils.

Alternatively, you can easily import custom field data by setting up a spreadsheet, adding columns for Forename, Surname, Gender, ideally UPN as well, followed by the custom field and any other information you require.

In the 3rd step of the import, Map Data, set the Column Type to Custom and you will see a second dropdown appear that will let you choose your custom field name.