The SOL that stays in an account
The third column on our worksheet is the one visitors resist. They have learned, sometimes in a single afternoon, to find the signature fee and the priority fee. Then a wallet screen shows SOL that cannot be sent away without closing the account, and they file that SOL under fees as well.
That held amount is the rent-exempt balance. An account on Solana stores data, and the account keeps a minimum balance tied to how large that data is. The balance is what lets the account remain. It is denominated in SOL, which is why it gets confused with the network fee, but it was not spent to process a particular transaction.
Left column, right column
At the desk we put fees on the left and the held balance on the right. Fees leave in the course of processing: the 5,000 lamports per signature, and a priority fee if one was attached. The rent-exempt balance stays with the account. When someone closes an account, that reserved SOL can become available again, minus the processing fee of the closing transaction itself. We say that sentence slowly, because it contains both ideas and people tend to keep only the half they heard first.
The exact minimum changes with the size of the account. A small token account and a larger account do not hold the same reserve. We do not ask visitors to memorize a formula. We ask them to point at the figure their own printout or their account view is showing, and to write it in the third column with the words held, not fee.
Drawer C
Drawer C in the reading room holds short notes on rent-exempt balances, written in our hand from public account examples, with the addresses trimmed off. They are there for the moment when a visitor says the fee and the stuck SOL are the same thing. We set a note beside the receipt and fill both columns from two different sources on purpose, so the page itself argues that the numbers have different jobs.
If you want this marked on a transaction you brought, reserve the window desk. Bring the printout. Leave the keys at home. The column only works if we can see the figure you are asking about, and if we are not also being asked to hold a secret.