This code system http://hl7.org/fhir/goal-relationship-type defines the following codes:

CodeDisplayDefinition
predecessorPredecessorIndicates that the target goal is one which must be met before striving for the current goal
successorSuccessorIndicates that the target goal is a desired objective once the current goal is met
replacementReplacementIndicates that this goal has been replaced by the target goal
milestoneMilestoneIndicates that the target goal is considered to be a "piece" of attaining this goal.
otherOtherIndicates that the relationship is not covered by one of the pre-defined codes. (An extension may convey more information about the meaning of the relationship.)