Abbreviation: F & R = "F&R Aspherical" (Tokina)

The abbreviations means "F&R Aspherical"

Explanation from Tokina

This lens, the AT-X 235 AF PRO encompasses Tokina’s new F&R aspherical molded glass elements of an exceptionally large 50 mm diameter in the front and 20 mm diameter element at the rear. The 50 mm front element is one of the largest aspherical elements being employed in optical designs today. These give outstanding performance with very even illumination in the corners and correction of spherical aberration across the image area.

See links

tokinalens.com