A packet submission (packet sub) tournament is a tournament that requires many participating teams to write questions (usually an entire packet, hence the name) if they are to play.
Packet submission tournaments, which were once the norm for how collegiate quizbowl worked (outside of SCT and ICT), have almost entirely disappeared apart from the ACF tournaments and Chicago Open. The packet-submission model has virtually never been used for high school tournaments.
The packet-sub model is usually contrasted with a housewrite, in which one or two teams produce all questions for the tournament, though that model is also declining in favor of larger writing/editing teams with players and alumni from many schools.
Packet submission tournaments are advantageous for editors because they disburse the work load for writing a tournament, so the central editing team can obtain a large number questions without having to write many from scratch (at least, in theory). They also allow lots of teams to influence the contents of the tournament (and the canon) more broadly) by writing questions on topics of their choice, which many good players enjoy doing.
In order to incentivize the submission of early packets, most packet-submission tournaments give large discounts to teams that submit their packets earlier, which diminish as the tournament gets nearer, and increasing penalties to teams that submit their packets late.
Due to the very real possibility for repeats between submitted packets, packet-submission requirements usually require between 24 and 26 submitted tossups and and an equivalent number of bonuses, though the final packet usually contains 20/20 tossups/bonuses. (ACF Fall 2004 asked for packets of 30 tossups and 30 bonuses, believed to be the modern record.) Each packet must follow a further subdivision into specific subjects and subcategories; for more information on the typical subdistribution for packets; see the ACF packet guidelines here.