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Taking hits along defined line

Hi all,
I seem to recall a way of defining a line a specific distance away from the origin (let say 100 mm along X), then, holding down the hit button on the Romer arm, scan across that line with the hits being taken only when it crosses that line. (hope that made sense).
Can anyone help me with this? Currently using 2013 MR1 SP7.
I can't remember if clip planes were used, but they stick in my head for some reason. Thanks