Shell behavior is still under development. One of the feature is to get the instance's expression(x,y,width,height,angle...etc), from uid.
Currently, it provides to get these expressions
Example of getting instance's expression from uid
<img src="http://i1081.photobucket.com/albums/j352/rexrainbow1/screen1-16.png" border="0">
In example, there are two kinds of sprite -"SpriteA" and "SpriteB". In line2, the global variable "uid" saves the uid of clicked instance of "SpriteB".
Then in expression of "action:append text", it use "SpriteA.shell.X(uid)" -- The SpriteA's shell behavior can get the position x by uid, even the uid is not point to SpriteA.
BTW, expression of shell behavior like "shell.X" actually accepts two parameters. First is uid, second is a default value if the uid doesn't point to any instance.