Find an apprenticeship master to attach to an apprentice. Results are grouped by normalised email address: one entry per person, with recordCount saying how many EXTERNALS records sit behind it — a school can hold dozens of records for the same address, and listing them all would make the screen unusable. Records with no email address are never returned: they cannot be reached, and matching them together would hand one person another person’s apprentices.

Pass companyId to list the people attached to ONE company — this is how a company sheet shows its apprenticeship masters, the mirror of GET /students?companyId=. The filter applies to the records BEFORE they are grouped, so recordCount then counts the records of that person attached to THAT company, and the entry returned is one of them.