There are 4 types of player acquisition system on ESPN Fantasy Football.
(This setting is locked at start of draft)
All unsigned players are put on waivers immediately following the league's draft. This happens from the start of the first game until a few hours past the end of the last, and when players are dropped from another team. When the waiver period expires, usually around 3 am ET, all the claims from that period will be processed based on the waiver order.
Players on waivers will remain so for a set period of time (Waiver period), during which all teams will have the opportunity to make a claim on any player. When the waiver period expires, the player will be awarded to the team with the highest waiver priority that made a claim, and that team will move to the end of the waiver order. This process continues until all waiver claims are processed after which all players not added via waivers become free agents which can be acquired by any team on a first-come, first-served basis.
Free Agent Budget (FAB) (Continuous)
Teams will have a specific budget to use for player signings. All offers will be processed in batches one or more times each week (as specified in the settings). If a team would like to add a player, that team will offer the salary amount they are willing to pay. On the next waiver process day, the team with the highest offer will be awarded the player, and the salary amount will be subtracted from that team's budget. All unclaimed players must go through the waiver process in order to be picked up by a team.
FAB waiver order is determined the same fashion as non-FAB, except it will be called "Free Agent Budget Tiebreaker". If you win a waiver claim, whether by bidding more or by winning via higher priority, you will be moved to the bottom of tiebreaker.
Please note: Selecting this system means ALL players not on a roster will ALWAYS be on waivers. Managers will not be able to make game-time roster additions. League Manager would be the only person being able to add players to teams via LM Tools.
Free Agent Budget (FAB)
Mainly same with FAB continuous with one glaring exception:
Players who are not claimed during offer processing become free agents available on a first-come, first-serve style pickup basis.
Every player is immediately available to be added to your team as a free agent once they enter the candidate pool or dropped from a roster or undrafted. Most leagues choose to utilize waivers instead of No Waivers as this system is often deemed unfair.