Fidelity Core Position

I’m about to open a Fidelity brokerage account but I’m not really understanding the core positions. It seems like SPAXX and FZFXX are the same thing. Are either recommended over an FCASH core position? I don’t plan to let money sit in the core position but if it happens I want to know which is best. #investments #personalfinance #fire

Also would love to know what everyone usually uses