What is the difference between a seal coat and a flood coat?

An epoxy seal coat is applied early in a project to prepare the substrate for the protective flood coat finish at a later step. In other words:

A seal coat prepares your substrate for the rest of the project.
A flood coat is the protective main layer of epoxy that covers the entire substrate and cures into a smooth, resilient barrier with a signature glasslike appearance.

What's an epoxy seal coat?

A thin layer of epoxy applied directly to the substrate using a paintbrush.
It's made using small batches of epoxy at a time, since you won't be pouring and the coat takes longer to apply.
The purpose of a seal coat is to prepare the substrate by sealing holes in porous materials like wood. By sealing the substrate, you prevent air bubbles from emerging from within the porous material during the flood coat stage. This makes it a lot easier to get the clean, glasslike transparency of when your flood coat cures.
It's also used to seal any small objects (e.g., bottle caps) into position before the flood coat.
The flood coat stage comes after the seal coat stage.
Learn how to apply an epoxy resin seal coat here.

A thick layer of epoxy poured from a mixing contained directly onto a substrate. The substrate will normally have a seal coat at this point, but some projects may not require a seal coat.
The flood coat will naturally self-level to 1/8 of an inch, without direct interaction.
Flood coats are applied one layer at a time. Usually only one is needed, but additional coats can be added four to six hours after each previous coat.
If you think you need another coat, but more than six hours have passed since the previous flood coat, you'll need to do the following:
- Wait a full day
- Then, lightly sand the surface with 220 - 320 grit sandpaper
- Wipe away the dust with a clean lint-free cloth and denatured alcohol or acetone.
- Caution: Do not use tack cloth or mineral spirits.
- After clearing the surface, you can pour another flood coat.