It means you're not a full time driver. As such, you are being coded as if you had a temporary status change from whatever your regular job and pay rate is, and that temporary status change pays you at or above pay guarantee depending on how many hours you work. You're getting coded as 4C (ground) or 4E (air). The equivalents without pay guarantee are 3C (ground) / 3E (air). Obviously since you're above the guarantee, there would be no difference in pay between 3C and 4C, or 3E and 4E.
If this is incorrect, talk with your FT supervisor so they can talk with HR.