F refers to Section F of the Federation Aeronautic Internationale' Sporting Code.
Sections A, B, C, D, and E cover man-carrying airplanes, balloons, parachuting, spacecraft, etc.
Paragraph 1 covers vol libre,
Paragraph 2, vol circular,
Paragraph 3, telecommade,
Paragraph 4, scale,
Paragraph 5, radio shack.
The lower case letters a, b, c, & d designate individual event; speed, stunt, racing, and combat.