The recordId on a SampleBottle is the foreign key that binds the container record to a specific CoCRecord. Since a single CoCRecord covers one well, recordId tells you exactly which well's sample is in this bottle. When you add a SampleBottle in Piper, you select the parent CoCRecord first, and recordId is stored automatically. This link is how Piper knows which bottles to list when displaying the full custody form for a specific well's sample.
Technical section
Data model, API, and internals — visible to developers.