If i would develop a ROS TCP/IP Client Node with the appropriate socket would it be possible to connect and communicate direct to the non-ROS positioning application Server and ask for the positioning data?
The short answer would be: yes.
This is done all the time, and we typically call these kinds of ROS nodes bridges (when connecting pieces of software) or drivers (when connecting ROS to (robot) hardware).
Be sure to check whether standard messages such as geometry_msgs or TF2 can be used to represent the location of the tag instead of creating a new custom message. Using standard messages will make consumption by existing nodes much easier.
Edit:
I hope it works because i found the rosbridge_suite a bit complex for this task.
rosbridge could also be used, but it depends a bit on how your "non-ROS application" works.
If it's under your control, and you can extend it with rosbridge compatibility, that could be a very lightweight way to be able to publish data to a ROS node graph without linking your/the "non-ROS application" to anything ROS (ie: roscpp).
If it's not under your control, and it only exposes some tcp:// endpoints, then rosbridge would not really help that much.
Actually the rosbridge_suite is useful only for Web applications?
No, not at all. It's just a simple JSON based protocol with a server which happens to "understand" ROS, allowing it to function (almost) as a transparent bridge and thereby allowing client applications to (almost) directly publish, subscribe and more without directly depending or importing any part of ROS itself.
I mean, if i wanted to publish the position of the tag into some web page then i had to use the rosbridge_suite, right?
You could use rosbridge for that, yes. But that's mostly just because there is a well supported client implementation of the rosbridge protocol written in Javascript.
There's nothing inherently "web based" in rosbridge.
