This option appears, if Railing's Segment is divided into multiple segments.
A Segment is the part of the Railing between two Nodes. So If you are placing your Railing associative by clicking on the starting and endpoint of the stair, the entire Railing - no matter how many 'change of directions' are in the railing - will be considered as one Segment. In this case, this command can be used: applying the command means that at every 'turns' a Node will be added to the Baseline and you will have more than one Segment.
I hope this answers the question! Please let me know if there is anything unclear!