It's nice to see you again here in the Community. Allow me to help share some information about adding a sub-category when printing checks in QuickBooks Online.
At this time, being able to add a sub-category when printing a check in QuickBooks Online isn't an option for us. The details available on the checks are the amounts and it's description only. However, the sub-category is applicable when printing the sales forms (Invoice, Estimates, and Sales Receipts).
For more information about managing categories in QuickBooks Online, please refer to these articles: