How do seeding and the start batches work?

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Accepted Answer

From the official aQuellé Midmar Mile FAQs and event information:

Events 2-8 have five start batches, in place for water-safety reasons, so that swimmers of similar pace start together and the field is spread evenly. Each batch has its own cap colour.
Each batch is led into the water before its start gun, and each batch leaves 3 minutes after the one before.
You're seeded on your entry time for that event and the total number entered in that race, so your batch can differ between events (for example blue in Event 3 but yellow in Event 5).
For Events 5-8, seeding is done once online entries close; late entries go into one of the last two batches (green or white caps).
You must stay in your allocated batch. You may move only to a batch behind yours, never ahead.
In Events 7 and 8 (ages 14-30), the top 20 seeded women and men are notified personally and have a dive start from the floating platform.

A time from an official seeding swim generally puts swimmers in an orange, blue or yellow cap, but the final batch isn't known until seeding is done about a week before the race.