An ID Key allows to store custom values on a per object basis. The Id Keys system got a major update.
Until now there was only one ID Key by default (Initial Transforms). Now there is a second one (Index) which can be used for all kinds of things. E.g. it can be used to sort objects before passing them to Animation Nodes. There are a couple of operators that help with that.
It is possible to create a new ID Key now. Note that it might disappear after a restart when it is not used anywhere. Furthermore some ID Keys can be removed as well. Wether a ID Key can be removed depends on where it is used. It cannot be removed when a Object ID Key node uses it or when it is one of the default id keys.
Animation Nodes currently supports 4 different types of ID Keys: Transforms, Text, Float and Integer. All of these can be accessed using the Object Id Key node.
Similar to how Transforms ID Keys can be loaded from the current transforms of an object, integer ID Keys can be generated in multiple ways.
Other options includes alphabetical sorting of object.