A spec sheet (or "spec book" for larger projects) is the working document that translates design decisions into orderable items. Every wall paint with brand, color, and sheen. Every fabric with vendor, pattern name, and yardage. Every fixture with manufacturer, model number, finish, and dimension. Without it, the designer's vision can't be reliably executed by anyone but the designer.
For consumers, the equivalent is a project document — a Notion page, a spreadsheet, a Google Doc — that captures every decision before purchase. This sounds like overkill for a single-room redesign but pays off the moment something arrives wrong, gets discontinued, or has to be matched later. "What was that paint color we used in the living room?" becomes searchable rather than guessed.
Good spec sheets also capture lead times. Made-to-order furniture can take 12-20 weeks. Custom upholstery can take 8-14. If you don't track these, the project schedule becomes the longest individual lead time, surprising you at the end.