Mostly, it's "whatever needs to be there to get the numbers to line up". Think of it like the mystery meat of your finance page
It tends to come out when there's a mismatch between your starting balance/previous balance, and our currently calculated cash on hand. Generally speaking, it shouldn't happen, but mostly likely is related to any sort of cash transactions (buyouts, cash from trades, etc...) which can potentially get put into the wrong columns.
If you can figure out where it comes from, let me know and we can see how to allocate it properly. I'm not saying it comes from a bug, but ideally that line item should never show up