By default, ASM has a system option enabled called ``Automatic Litter Identification''. This means that any new litters you create will be automatically assigned a unique identifier.
A button is available on the edit animal toolbar allowing you to assign an animal to a litter. This populates the Litter ID field on the animal to show that it is part of the litter.
Once this is done, you don't need to do anything further, as ASM will watch for the animals leaving the shelter and automatically complete the litter for you.
This method is the most recommended and works in the exact same manner as the ``acceptance numbers'' method outlined further below. They are in fact the same, however acceptance numbers only apply to RSPCA shelters in the UK, whereas this mechanism is generic for any shelter in the world.