Invoice id creation methodology

I am approaching the orders and invoice creation.
I would like to know ho is generated an
Is it always linked to an order?
Can be changed (in case we restart from a different number?) or customized (example: FA/1 is for a service while FZ/123 is for a different service purchase)?

Order and Invoice id’s are auto incremented and created in the IS database. The first invoice id for an order is the same number as the order and each successive invoice (in the case of recurring invoices from that same order) are incremented by 2 each time it’s generated. There is no way to set them to anything different.