This is the number you may have been assigned if you have had past immigration history and interaction with USCIS for various immigration benefits.
Should I enter this number?
Yes, if you know that you have an A-number please enter it here. There is a lot of misinformation about this topic but as per USCIS guidance if you have an A-number and know what it is you should enter it here.
I have an A-number but cannot find it, what should I do?
If you have an A-number, you should be able to locate it on past notices and other documents you have received from USCIS. If you cannot locate this number please leave it blank.
Will USCIS assign me a new A-number if I don't have one or have had one previously?
Yes, USCIS will automatically assign you a new A-number if you don't have one. Typically, you might find that USCIS automatically appends your existing A-number to your application if you don't enter it. There is a possibility that you may get a new A-number assigned to you even if you had one previously.