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Marks Tab>Edit Marks then in the Marks Mgmt window Open a New Route on the left, mind your previous Marks on the right and use the << to move them into you new route (subset)

That's one way...

You can also edit the original Route marks as Always Draw then when you want the subset you can use the Visual Marks feature and add the marks off the chart.

I use both ways, pretty easy.
Cheers,
Ernie