I thought nothing of it, but when I set-up DW's Fidelity, she got an 'extra' zero balance account. I didn't review the whole thread, but a few posts made me think this story might be relevant here.
DW had a tIRA and Roth for a long time at Fidelity, but no brokerage account and no money market account. She got a windfall sum that she didn't want to put in her normal spending account (gets no interest), so I opened a Fidelity brokerage account.
When "done", under the section "Investment Accounts", she now has "INDIVIDUAL - TOD" with a cash position (SPAXX**) and an equity position.
But also, and something I didn't specifically ask for, under a section called "Savings, Checking and Spending Accounts" is another "INDIVIDUAL - TOD" and it has a position "CORE**" with a description of "UNFUNDED CORE POSITION".
I'm not sure why this 'extra' account was added, but I didn't see that it was doing any harm.