There are a number of ways that the caller can identify which dancers should trade. One way is by their position. For example, in an Ends Trade, the end dancers will drop hands with the adjacent center dancers, and then walk individually to the other end of the line.
For a Centers Trade from any kind of line where the centers are facing in opposite directions, the dancers will drop hands with the adjacent end dancers, but will keep holding hands with each other. As they walk into each other's spot, they will be doing the equivalent of an Arm Turn 1/2.
The dancers don't have to both be ends, or both centers. They can also be identified in other ways, such as Boys vs. Girls. But they must be in the same line as each other. They can be close together or far apart, but they will always be directly left or right -- never forward or back.
For a Boys Trade, just the Boys trade places with each other -- the Girls don't move.
For a Girls Trade, just the Girls trade places with each other -- the Boys don't move.
If the two dancers who are trading start facing the same direction (the same wall), they will need to pass by each other, on the same side of the line, on their way to their destinations. In that case, they pass right shoulders. Note also that, since they started facing the same direction as each other, and they each turned around halfway, they will end facing the same direction as each other.
Regardless of the type of Trade, the following rules always apply: