In addition to the ID that is shown in the dropdown, in order for a field to appear there it must be one of the following:
A field of the Number Auto-Increment type
A field that has the Must be unique box checked
Because of the way Knack's authentication works, you choose a specific application when you connect Knack to Zapier. Since we need the Application ID to connect to Knack, we aren't able to show objects from multiple Knack apps in the same dropdown. The objects from the connected app are the only ones you'll see when you're creating a Zap.
If you need objects from a different Knack app, just make a new connection to your Knack account, and that will allow your Zap to show the objects from that newly-connected app.
Knack only triggers notifications from actions that happen in your app, not actions directly from the database or through the API.
When inserting records, only records from form submissions will trigger a Zap. Records from form rules will not.