What are ExecSeqMarkPos and ExecSeqRestrPos used for

Hi,

  I'm reading code of nodeSeqscan, and was confused with ExecSeqMarkPos and
ExecSeqRestrPos. They are only called by ExecMergeJoin. Could merge join use
a plain seqscan as outer/inner plan? If not, what are they used for?
  
  Thanks.