FieldService:n should be used in the “project-scratch-def.json” file to have a Salesforce Scratch Org with Field Service Lightning feature. Here, n represents the number of Field Service Lightning Licenses needed in the Scratch Org.

Sample project-scratch-def.json:

{
  "orgName": "Testing FSL",
  "edition": "Developer",
  "features": [ "FieldService:1" ]
}

Here FieldService:1 gives 1 Field Service Lighting License. If you need more, then please use FieldService:5 for 5 licenses.