Here’s a clear-as-mud explanation I found:
Most games are selected with either 12-day or six-day notice to the Conference. The networks each have two opportunities (four total) to select games on six days notices annually. On or before June 1, ABC/ESPN and FOX will pre-select games for the first three Saturdays; special-date games (like Thursdays), games on Thanksgiving and Thanksgiving Friday. For the remaining weeks, ABC/ESPN and FOX shall select games no later than Noon CT on the 12th day preceding a Saturday telecast. ABC/ESPN and FOX have the right on four dates to make the final selection no later than Noon CT on the sixth day preceding a Saturday telecast. Generally, homecoming games will be selected at least 12 days in advance, but the Conference must consider an occasional request to use a six-day pick to select a homecoming game of significance