Set an alternate for one part on one BOM line. A BOM substitute applies to a single entry in a single project, and nowhere else. Use it when a part works in this design but you would not approve it everywhere: a customer-sanctioned alternate for one build, a part that fits this board in particular, or a one-off swap to get a batch out the door.
A meta-part groups parts under one name for every BOM that uses it, and a part substitute applies wherever that part appears. A BOM substitute changes nothing outside the one entry you set it on, so the part keeps its identity in every other project.
The distinction matters in practice. An op-amp may be replaceable by a cheaper one in a temperature-controlled product but not in the one that lives outdoors; a customer may approve an alternate connector for their product only. A global approval would be wrong in both cases — the approval belongs to the line, and a BOM substitute records it exactly there. Open the BOM entry, add the substitute part, and the rule is stored with the project.
The substitute part itself shows the connection: its "used in projects" list includes projects that reference it as a BOM substitute, so an alternate never becomes an untracked shadow inventory.
When you price, purchase, or build that BOM, PartsBox combines all three levels — the entry's own substitutes, the part's substitutes, and any meta-part members — into the complete set of parts that may fill the line. It gathers offers across the whole set and picks the best, so a per-line alternate can win on price as readily as it covers a gap. During a build, stock of the entry's substitute counts toward the line, which is how a shortage on one part stops blocking the batch.
A substitute must share the part's unit category, so quantities stay comparable — a part measured by length can only be replaced by another measured by length.
BOM substitutes are in every plan, including the free Maker plan.