At the top of the part there is a small gap that we need to close. We will use the Join feature to do this. Join will operate on gaps, overlaps, overshoots and crossed ends. The segments may be lines or arcs.
A distance value is used by PrimeCut to determine whether or not to join segments. If the end points of the segments are within the value then PrimeCut will attempt to join them. Generally this will be done by extrapolating or shortening the line. If a corner is being joined, then the lines may be nudged slightly if an extrapolation would extend the lines excessively. This is illustrated here:
Nudging may be acceptable, or even desirable in some instances. This will depend on the nature and requirements of the part. Where nudging is undesirable, a 'corner join' may be more suitable.
If only one segment is selected, then PrimeCut will attempt to join it to the segments before and after. It will continue attempting to join subsequent segments until it reaches a segment that cannot be joined, or it has closed the loop.
If a number of segments are chosen, then PrimeCut will attempt to join only those segments. If at one point it is not successful it will still attempt to join the subsequent segments. This is the technique we will use here. By selecting only two segments, only one join will be attempted.
1Use the Zoom function to have a closer look at the gap at the top of the part:
2Click the Select all Snaps button to enable all snapping options.
3Click the Measure button. 
4Click on one line end point, then move the mouse to the other end point:
Measurements are shown at the top left of the editor window. In this case we can see that the gap is 0.5212mm long. If we are to join these segments, our tolerance will need to be greater than this measurement.
5Select both of the segments using a 'crossing rectangle' (from right to left) and right-click to view the popup context menu.
6Select Join from the menu. The following dialog will be displayed:
7Enter 0.6mm / 0.0236" and select the check boxes as shown. Click OK. The segments are now joined.