Bridge Segments are a concept that help to make your soundtrack manageable when it comes to transitions from one Group (or Theme) to another. You can route your audio flow through these Bridge Segments upon those transitions, so that you gain more control and avoid transitions to other Segments that are likely to sound awkward in that context.
Here's an example. E.g. let's say you composed two Themes, one ambient music for a Cave and one for a Forest, and you provided loads of Segments for both Themes. While all those Segments might work very well as long as you stay within the same Theme, the transition from Cave to Forest might sound too abrupt or dissonant in many cases. In fact there might be only very few Segments in Theme Forest that are suited to be played directly after most Segments of Theme Forest. Without the concept of Bridge-Segments you would have to manually block all the transitions from all Cave to all other Forest-Segments. Even worse, you would have to keep that in mind for every other Segment you might add later to Theme Cave, and block that too. The solution to that problem is to declare the well-suited Segments in Theme Forest as Bridge Segments to Theme Cave. You do that by first selecting the Source Group in the Source Tree View, which in our example would be the default group of theme Cave. Now you make sure that the Forest-Segment suitable for the transition is visible in the Target Tree View, and right-click that Segment. Its compatibility is now shown as "Bridge", and the compatibility of all other Segments of theme Forest's default group is shown as "implicitly blocked".
Automatic Bridge Segments
In the last section we learned how to define a certain Segment within a Group as a Bridge Segment for a given source Group. Now there are some Segments that generally work well for all kinds of Group or Theme transitions, like those starting with drum-crescendos or swoosh-effects. In this case we might decide to use this Segment generally as a Bridge Segment for transitions to its Group, no matter which Theme or Group was played before. To achieve this, we can set the "Automatic Bridge Segment" flag in the Segment Property Panel. This is especially useful when multiple composers are working on a project, as the Automatic Bridge Segment flag makes it obsolete to manually define that Bridge Segment multiple times for all groups after importing Themes.