Formulas: calculated fields
A Formula_ sheet (the plural Formulas_ and Logic_ also work; Formula_ is preferred) adds calculated fields — values the app works out for you rather than asking someone to type. Put one formula per row across these columns: Output Field, Formula, Description, Section, Form. The output field is read-only and updates live as the record is filled in.
Refer to fields by name. A total that adds three fields is just hotel + meals + transport. In a LINE ITEMS section a formula can total a column into a header field — sum of amount — and reference a header value with a readable qualifier: miles_driven * mileage_rate from HEADER.
calc rule. A Formula_ field is worked out in the browser and shown as you type — great for totals and on-screen logic. A calc rule (on the Rules sheet) is a currency/unit conversion the server computes and stores. Reach for a formula for arithmetic and IF/Boolean logic; reach for calc to convert money or units.Formulas aren't only arithmetic — they can make decisions. Use IF(condition, value_if_true, value_if_false) and Boolean logic (AND, OR, NOT) to pick a value from the data, e.g. IF(total > 100, "High", "Low").
See the Formula reference for every operator and function, with worked examples.