To further expand my confusion.
Tiller’s support documentation states that:
“If the transaction is already categorized or AutoCat has already made modifications (as indicated by a value being present in the “Categorized Date” column) by default AutoCat will not re-categorize or modify the transaction again.”
When I manually set a category for any transaction, that does not change or add any values to the “Categorized Date”, hence AutoCat should still process that transaction as long as it meets its criteria.
Furthermore, as per another community post about Tags and AutoCat:
“For a transaction to be processed in AutoCat, every non-blank filter column criteria must be met.”
This makes me believe that it is perfectly fine to create rules like the one I shared in my original post, where I can filter by existing category and apply the corresonding Tag. There is no need to add/set a Category in the first column.
Even the Advanced menu to create AutoCat rules allows me to do this without errors.