All of the ASI forms use AST:Attributes as the secondary form, using Reconciliation ID as the join criteria. The UI on these forms continues to be used as the exposed UI.
Other BMC Remedy ITSM joins with BMC.CORE:BMC_BaseElement, that use attributes moved to AST:Attributes, are rejoined with AST:BaseElement.
Design of ASI base CI (does not have UI exposed), AST:BaseElement
Design of CI with UI exposed, for example AST:ComputerSystem
Design of BMC.CORE:BMC_BaseElement joins using AST:BaseElement