Hi Jay,
Yes one order type may have multiple categories. 1F will have categories other than planned orders like Stocks & Distribution orders etc, Each categories may grouped to form a category group. Categories are every single KF you're going to plan in APO
Order Type is how you're to perform the planning (Replenishment/Requirement Planning)
1F - In House Production - where the the replenishment is done by producing the requirement (materials) in the Plant owned by company as per your demand. Thus Planned Orders are created --> Converted to Production Orders --> Production execution --> Stocked-->Fulfilling the demands.
External Procurement - where the the replenishment is done by procuring (Purchasing) the requirement (materials) from external suppliers. Thus Purchase Reqs are created --> Converted to Purchase Orders/SA --> Received from Supplier --> Stocked-->Fulfilling the demands.
Explore the Planning type and categories associated with it you'll get fair knowledge.
Go through SAP Help/Library and SCN you''ll find interesting and informative info.
Good Luck n Great Weekend to all
Regards
Vinoth