Contacts
Suppliers, service partners, and the people you lend to. Home → Contacts.
Adding one
+, then choose whether this is a company or a person, and fill in what you have — name, role, email, phone. Nothing but the name is required, so a contact captured on a site with a phone number and nothing else is perfectly valid.
A person can belong to a company.
Linking a contact to equipment
From an item: More → Suppliers & service. Pick the contact and give it a role — supplier, manufacturer, or service partner.
That is what makes "who services this compressor" a question the person holding the compressor can answer.
Plan
Contacts is a plan feature. On a plan without it you can still read the contacts the account already has — the list opens, and says why the add and edit controls are missing.
What stays in the web app
Importing contacts from a spreadsheet, merging duplicates, documents and the full activity view are web app work.