BpDirective Types BO PreValidation?

Just noticed this in the BPDirectiveType table

Source DirectiveType Name SystemFlag
BO 0 PreValidation 1
BO 1 PreProcessing 1
BO 2 BaseProcessing 1
BO 3 PostProcessing 1
DB 0 InTrans 1
DB 1 OutOfTrans 1
DQ 1 PreProcessing 1
DQ 2 BaseProcessing 1
DQ 3 PostProcessing 1

What is Prevalidation? I don’t believe this is user accessible.