Prism is fully configurable for inches (imperial/standard units) and metric units.
Every Prism user gets to choose whether to work with inches or millimeters. The setting is made on the Account page on the iPad app or web app; the same setting is used for both. When set to millimeters, all values will be displayed and edited in millimeters, and similarly for inches.
A choice of units is also given when exporting G-code, regardless of what was used during programming. The default choice of units for G-code is controlled from a setting on the Machine settings page in the web app.