You can predefine keywords/phrases, yes. But there are many other options. You can tag different documents based on how they wherer ingressed (which e-Mail they came/were sent to from for example).
I have it set up so that my scanner has a few different quick action buttons which atomatically upload the documents into different folders (think bills, helthcare, bank, etc.) Then paperless tags and sorts them based on those folders.
I also does machine-learning when enabled which works ok in my experience.