DefaultDimension in d365 FO X++

 Navigate to PurchTable methods you can find with DefaultDimension

this.DefaultDimension = this.mergeDimension(this.getDefaultDimension(),this.DefaultDimension);

this.DefaultDimension = this.mergeDimension(_purchAgreementHeader.DefaultDimension, this.DefaultDimension);

!DimensionDefaultFacade::areEqual(this.DefaultDimension, _purchTableOrig.DefaultDimension))

this.DefaultDimension       = this.copyDimension(_purchTableHistory.DefaultDimension);