A template controls how a bid or invoice looks: logo placement and size, accent color and font, which columns and sections appear, header fields, notes and terms, the footer, and the paper size. Nothing on a template changes your numbers, taxes, or line items, only how the document is presented.
Each company can save multiple templates per document type, Bid, Invoice, or Receipt, edited separately, and one is marked the default for that type. Every kind always has at least one template, a “Standard” one that reproduces the classic look, created automatically the first time you visit that kind’s tab; it cannot be deleted while it is the only template of its kind.