What functions/methods can you use in the bad end expression language?

I have been trying to track down what is possible via the back end expression language in Flowable but I am struggling to find any concrete information. I have found the following.

Does Flowable 6.5 use JUEL, Jakarta? it is hard to find out from the documentation.

I am asking from an implementation perspective. I am implementing BPMN definitions and trying to find out what I can and can’t do with the expression language within the workflows.

I have stumbled across things like .concat and .toUpperCase but unsure what else there is.