A named range gives an address a readable label. After naming B2:B5 Actual_units, a formula can use SUM(Actual_units) instead of SUM(B2:B5). Choose a name without spaces or a cell address. Names belong to the spreadsheet and can refer to a particular tab.
Open Data > Named ranges to create or update one. Check the tab and both ends of the range; a convincing name attached to the wrong cells still gives a wrong answer. Changing a label in A1 doesn't rename a range.
Protected sheets and ranges control editing. A warning-only rule asks someone to confirm an edit but still allows it. A restricted rule prevents edits by people outside the allowed group. Protection is useful for formula cells and agreed targets, but it isn't a password or a way to hide confidential data from someone who can open the file.