Afterbirth+ introduced 1 more, as well as an additional mark for Greedier mode, bringing the final total to 20. Afterbirth introduced 3 more, bringing the total to 17, as Greed mode only has one mark. In Rebirth, there are 6 unique completion marks, and each one comes in two varieties, adding up to 12 marks in total. There are a few additional rewards for completing many or all completion marks. In Afterbirth and onward, every other regular character has a co-op baby that can be unlocked for earning all of their completion marks on Hard mode. Therefore, it is not necessary to earn every Hard mode completion mark to achieve Platinum God, only every Hard mode Heart mark, with The Lost as an exception.

In Rebirth, Mom's Heart/ It Lives is the only boss that has a character-specific co-op baby unlock if you beat it in Hard mode instead of Normal mode, with one exception - Godhead, which is unlocked by beating every boss in Hard mode with The Lost. If a completion mark is earned in Hard mode without first earning it in Normal mode, the game will count both the normal mark and the Hard mode mark as unlocked. If objectives are completed in Hard mode, the completion marks are outlined in red. If objectives are completed in Normal mode, the completion marks are solid black. As you complete these objectives, the post-it note on the character select screen fills up with Completion Marks. Completion marks are earned by completing specific objectives in The Binding of Isaac: Rebirth.

Binding of isaac rebirth symbols