TC Enqueue waits

Hi List.
I have detected high enqueue waits in one instance.
After quering x$ksqst I realized most of them were TX and TC enqueues.

I'm on the TX enqueues by tracing some sessions. But I have never heard of TC 
enqueues
I have read that TC enqueue stands for "Thread Checkpoint enqueue". 

Anybody knows what are they used for? what kind of resources they protect?

And I have another quick question:

I'm also seeing Parallel query events like:

PX Deq: Execution Msg
PX Idle Wait
PX qref latch 
PX Deq: Execute Reply 
PX Deq: Table Q Normal 
PX Deq: Parse Reply 

Does anybody know where I can get a description for them? 
I'd like to know which ones can be ignored and which ones can't.

Thank you very much
Regards
Diego
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