Edit > Edit Piecemarks ( Classic, Modeling & Drawing Editor )

Combine / Break Apart Opens a list of members. Select from that list those members you want to be assigned the one user piecmark that you subsequently enter.
Remove Opens a list of members. Select from that list those members whose piecemarks you want to remove. This deletes associated details and allows new system piecemarks to be assigned.
Change from User to System Opens a list of members with user piecemarks. Select from that list those members you want to be combined under system-assigned piecemarks.
Change from System to User Opens a list of members with system piecemarks. Select from that list those members that you want to be user piecemarks.

Also see :

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