Allow Common-line is a tick box on the part's nesting properties (Orientations page). When it is ticked (the default), autonest may join the part into common-line strips or pairs. Untick it to keep the part out of common-line autonesting; it is then nested as an ordinary part.
The tick box is greyed when common-line autonesting is off for the machine. Turn on Autonest Common-line Arrays in the machine settings first.
The setting is saved with the part. With it unticked, C will not make the part a common-line array, but you can still glue it by hand with Glue Common.
Parts with no flat edge long enough are left out of common-line autonesting automatically, whatever this setting says.
See also Auto Nesting Options.