I would say a DIY investor does not pay anyone for investment advice and must submit all their own orders for transactions.
As for the benchmark to judge against, it will be different for every investor and would match in some detail their asset allocation. That means, the same percentage of US large-cap, US-mid-cap, US-small-cap, same for Int'l, and the different kinds of bond funds, too: long, intermediate, short, US government, corporate, TIPS, etc.
Whenever there is talk of benchmarks on this forum, folks either write, "I use index funds, so that's my benchmark and by definition I match it" or "It's too complicated, so I don't have a benchmark."
And that doesn't even go into the fact that probably 90%+ of investors do not know how to calculate the performance of their investments and cash.