How do you know how to categorize job supplies versus supplies and materials?

You should know whether what you buy is for a job, or for the business (office, shop, etc), use the expense account the payment pertains to.

But overall, they are both expenses so no matter which you use the bottom line on the P&L will be the same

So if I need to buy a $ 300 PDF with Safety Harness to deliver a yacht as a boat captain, would that be job supplies or office supplies?