Under Administration → Qualifications you create certificates, e.g. "High-voltage training level 2S" with 24 months validity. For each employee you enter in the user form who holds the certificate, since when and until when (empty = calculated from the validity). A tool (Loan rules & NFC) or a whole category requires a qualification – without a valid certificate the tool can neither be issued nor handed over to a colleague, not even via pick list or the API. Expiring certificates appear on the overview, under Qualifications and in the daily summary email.
Further rules: Issue only by tool issue or foreman (four-eyes approval) prevents mechanics from booking it themselves. Each category has a Maximum loan duration – it is set if there is no return date, and Tulz refuses a later date for mechanics. Under Settings → Workflows you limit the Maximum open loans per employee. Tool issue and foremen may exceed both limits.