Universal Robots are on the market in several revisions, known as generations.
The first generation was CB1 (CB= control box version), and was launched in 2008.
Soon came the CB2 In 2011. However in 2014 the CB3.0 was released, and got updated to CB3.1 in 2015 (for UR3) and 2016 (for UR5 and UR10). The latest generation is the e-Series, which was launched June 2018.

The e-Series robots was launched in June 2018, and exist in all three sizes; UR3e, UR5e and UR10e. 

CB3 was launched in 2014, including UR5 and UR10, while the UR3 joined the family in 2015. 

The URCaps SDK and API was introduced in PolyScope software release 3.3.0.
This software is compatible with any CB3.0 and CB3.1, which can be upgraded to this version.
CB1’s and CB2’s cannot be upgraded to support URCaps.
CB3.0 was originally launched with PolyScope software release 3.0.
However, this software can easily be upgraded to software 3.3, which will support URCaps.

The URCaps API is released along with a feature upgrade of PolyScope, hence to use a later version of the API, the corresponding minimal version of the PolyScope software must be used.
E.g. to use features from the API version 1.2, the robot must as a minimum run PolyScope 3.5.
Newer versions of PolyScope are backwards compatible with older API versions, hence e.g. e-Series PolyScope version 5.0 is compatible with any API version from 1.3 and older. 

 As the software is fully upgradeable within the same generation of robots, UR recommends always using the latest API, when creating a new URCap, so obtain the most rich feature set. 
Besides from varying features in the API, there are some hardware characteristics in respect to the OS, that differ between the generations:

CB3.0 or CB3.1?

All UR3’s are made with CB3.1, and all UR5’s and UR10 made from February 2016 also have CB3.1 controller.

If you need to verify the controller version, this may be done with serial number.
From the below serial numbers, the controlbox will be CB3.1 UR5: 2016351864 UR10: 2016301779
However note, that a CB3.0 to CB3.1 upgrade kit may be fitted later, hence this is not a unique validation.

By inspecting the controller physically, the CB3.0 has a CF flash card for file system storage, while the CB3.1 has a USB3.0 memory stick.