SYSML21 — state-actions-compartment-contents appears to imply that the item being reported is a relationship

state-actions-compartment-contents = (state-actions-compartment-element)* '…'?
state-actions-compartment-element = el-prefix? EntryActionMember | DoActionMember | ExitActionMember
EntryActionMember : StateSubactionMembership =
MemberPrefix kind = 'entry'
ownedRelatedElement += StateActionUsage
DoActionMember : StateSubactionMembership =
MemberPrefix kind = 'do'
ownedRelatedElement += StateActionUsage
ExitActionMember : StateSubactionMembership =
MemberPrefix kind = 'exit'
ownedRelatedElement += StateActionUsage

I am accustomed to seeing property += keyword to indicate that a relationship is to be followed from the current item to the item of interest. This only appears within the sub clauses to jump to the element owned by the membership. Thus I must assume that this compartment is shown on a symbol or line representing a SubactionMembership.
If this is incorrect then please correct the specification or offer an explaination.