Reference designator

R5. U1. C12. A letter for the kind of part and a number that makes it unique on the board.

It looks like a cosmetic label and it is closer to a primary key. The schematic, the netlist, the bill of materials, the layout and the assembly line all refer to the same physical component by this string, and nothing else they carry is common to all of them.

flowchart LR
    R(["R5"]) --> S["schematic<br/>the symbol on the sheet"]
    R --> N["netlist<br/>which pins join which nets"]
    R --> B["BOM<br/>part number, quantity"]
    R --> L["layout<br/>the footprint on the board"]

Two consequences follow, and both show up in this tool.

Comparing two revisions matches components by designator, so a part that keeps its designator is the same part and a part that gains one is a new part. Renumbering a board therefore reads as deleting everything and adding it back. That is a real property of the identifier rather than a shortcoming of the diff.

And a designator has to be unique, and duplicate-ref-des checks that. A board carrying two R5s has two parts that no downstream tool can tell apart. The exception is a multi-unit part, where several symbols on the sheet are one physical component and share one designator deliberately.

A part with no designator yet is unannotated. That is normal mid-design and a problem at handoff.

The course does not teach it yet. It names parts as R1 and U3 from chapter 1 onward without ever saying what the label is, and this page fills that gap.