Few fields are not available for Field update and to be added in Workflow conditions while configuring Workflow Rules.
Reproducing the Issue
Go to Administrator workcenter.
Go to Workflow Rules view.
Click on New to create a new workflow rule.
Select the Business Object as Ticket and enter all the mandatory fields.
Click on Next, under the Define Conditions click on Add Group to add a condition.
Here,for 'Field' select the dropdown.
Few fields such as Coverage are missing from the dropdown.
Only the header level fields of the Business Objects will be available for Workflow conditions or Field updates.Supporting fields at the item level with Workflow has several techinical complications.
This is the expected system behavior.You would have to use SDK to build this feature.
condition, update, standard field , KBA , item , update , LOD-CRM-WKF , Workflow , Problem