MailChimp merge fields can be mapped to any of the WP-Members registration fields in the mail merge fields settings area. This area will indicate all mail merge fields in your MailChimp list, whether they are required fields or not, and what WP-Members field they are mapped to.
Merge fields that are not required do not have to be mapped for subscription to work. However, only merge fields that are mapped to registration fields will have data passed to them when a user subscribes.
If a field is required in MailChimp, that data MUST be passed during subscription or the user will not be added. This means the field must have data associated with it. This means it should be mapped and that field should be required. (The exception to this would be if a specific piece of data is to be set for a specific merge field and that data is generated by a function hooked to the wpmem_mc_merge filter hook, but that’s a more advanced topic for special situations.)
There is a wpmem_mc_merge filter hook in the module for you to use and this can be used to programmatically set values to be tied to specific merge fields.