Nexus recognizes a marker's trajectory continuously throughout a captured trial. There can be gaps in the data, but Nexus can still recognize the same marker trajectory on either side of a gap, as long as the gap is small enough or the marker's movement doesn't vary significantly within the gap.
Occasionally, instead of a gap or a continuous marker trajectory, Nexus creates two separate sub-trajectory sections that belong to the same marker. This is called an overlapping trajectory, because the two sections overlap; that is, they are both present in the same frame(s).
During the labeling process, because both trajectories are present in the same frame(s), Nexus assumes that they must belong to separate markers. Therefore, when Nexus auto-labels these trajectory sections, it labels one and leaves the other unlabeled.
Overlapping trajectories can cause difficulties during manual labeling. One common difficulty occurs when two markers are correctly labeled at the start of the trial but then become confused or "swapped" later in the trial.
If you attempt to correct this label swap with Whole selected in the Manual Labeling section of the Label/Edit Tools (the default setting), you may lose correct labels early in the trial. The easiest way around this issue is by clicking the Backward or Forward button before labeling, so that labeling is only continued in the frames before or after the current frame.
When this labeling method is used, Nexus "snips" the trajectory at the current frame and only labels in the direction chosen (forward in time or backward in time from the current frame). The process of snipping a trajectory causes a single continuous trajectory to be split into two separate unique trajectories at the current time frame.