In Case-Subcase relation, pxCoverInskey is the Parent's case pzInskey which is linked to all the associated child cases. pxCoveredInskey which is of type value list contains all the associated child case's pzInskey attached to the parent case.
pzInsKey - Unique identifier of all type of cases.
pxcoverinskey - Indicates the pzInsKey of parent case for any subcase. It measn if you read this property for subcase it will same as PzInsKey of its parent. For parent case, the value of this property will be null.
pxcoveredinskey - Set of pzInskeys of all child cases of any parent case. If you read data for parent case you see something like this for value of pxcoveredinskey . C1 is the child case here.