relational files

hello,

this is my first week using FileMaker 5.5 at my company. i'm trying to make a database for my company's older/antiquated system. i need a little help with what seems like a simple solution.

i have a file about a certain customer that has done busines with our company many times; this customer has specific PO numbers for every job request he has made with us. all PO numbers in our database are to be ordered sequentially (1, 2, 3, etc...). that's easy to do. but how do i do PO numbers sequentially if i have multiple customers who don't necessarily 'take turns' doing business with us; i.e. customerA requests two jobs, then customerB requests one job, then customerA requests another job AGAIN, then customerC requests one job?

is this a situation where i use a relational field or a portal? how do i go about doing this?

thanks,

me