Hi MM,
These types of questions of "can the system do this or that" that tend to be vague and difficult to answer (there are so many possible answers, that can be right or wrong depending on the situation) . But let me see where I can go with this and may be trigger some clearer requirements.
On Workflows: In D365 F&O, in the Procurement and Sourcing Module you can have Workflow to approve Purchase Requisition header and Purchase Requisition Lines. Similarly, you can have workflow to approve Purchase Order header or Purchase Header lines. Procurement categories are typically assigned to PR and PO lines, so unless you limit PO Header and PR Headers to single Procurement Categories, you would have to split the PR and/or PO approval to line workflows. From there, you can go crazy and create as many workflows versions as there are categories (if you so wish).
I like simplicity; simple is good; most people can deal with simple. So I suggest the K.I.S.S. principle: dont go crazy. I dont know how many Procurement categories you have but I have rarely seen a company that has less than a hundred. Typically, approvals are made more in terms of organizational hierarchy or division, department, cost centers etc than along "categories" line.
Signing limits are available for these two documents and are normally used in workflows as a condition to <exit> the approval cycle. There are several types of Signing Limits, Purchase Order and Purchase Requisitions included. Each give you 2 amounts (spending and Approval). Typically you would have approval workflow go up an hierarchy from the {requester / preparer / orderer} to exit when the assigned approver has sufficient signing limit. You are not constrained to use the Organisation Hierarchy and can setup alternate hierarchies (dubbed "configurable hierarchies") with different reporting structure between positions; these configurable hierarchies can then be assigned to PR or PO workflows to be used.
I realize that this answer is generic, but if you can clarify what you are trying to achieve, may be we can dig deeper into specific options.
