Field IDs are internal references to fields, usually these are customised fields created to store values such as Moodle Category Ids, Reseller names or Moodle template ids. These values can be used in plugins or javascript files to refer to specific fields, referring to the ids means that it does not matter if the name changes, the id remains the same.

For example:

When editing or viewing the field Moodle Category Id, get the Field Id from the URL

https://ccl.test.coursesales.com/field/edit?FieldId=4764

In this example take the id “4764” and add that to the field CourseSales.com Course Category field holding the mapping to Moodle categories on the enrolments plugin form in Moodle.