Hello,
AC Field Name is the field of GRC System and System field is of Target system can be LDAP or portal etc.
Like email is field is GRC System but the same may be stored in other system as mail.
if you use LDAP.
USERID in sapsystem is but user id are stored in LDAP as SAMACCOUNTNAME.
all your details are stored in security account manager database in LDAP .
when you map field you need to do mapping.
like what field you want to get imported and which attribute to be read to update that field.
the same you can understand about the attribute in LDAP mapping .
Regards,
Prasant