In the Add new Customer page and Customer Detail Page you will find n/a on two fields.

You will find N/A on the Mobile field

and you will N/A on the Work Email field

The checkbox on these fields are to be used when the customer does not provide a mobile number or a work email.  The tells the system that the field is not available or not applicable.

The should be used if the customer does not provide you with a mobile number or does not provide a work email address. 

The is there to avoid typing generic details to fill in the required field.

If the customer does not provide you with a mobile number or a work email address press the related for the field not provided or given to you or you do not have it.