Once you've learned a Trust, you can cast it forever in that game. Each one technically still had a role like Tank/DPS/Healer, but in FFXI although some Trusts were the same jobs, they acted very different in battle. For example, you could unlock one White Mage trust that came equipped with a basic set of spells, and others that came equipped with more advanced magic. They function using a gambit system like in FFXII, but you can't edit their actions.