A field is not available for selection to set value of another field when creating a Business Rule.
Employee Central: Business Rule
Reproducing the Issue
There is a field "Custom Job Level' in Positions.
There is a field "Custom Job Level" in job Classification.
This field from Job Classification is not available in a Business Rule to assign to the field in Position.
The two fields are of different types. The field in Position is of type String.
The field in Job Classification is of type Picklist.
Both the fields should be of same type or of compatible type. To resolve this issue here, we change the field's type to Picklist in Position object's definition.
Now we can set this field's value from the field in Job Classification.
Type , KBA , LOD-SF-EC-RUL , DO NOT USE - Business Rules & Event Derivation , Problem