What do these article submission dates really mean?
This set of deadlines sounds like the conferences is doing two rounds of review for submissions.  


First, you send in an abstract in February, and they decide if it's worth reviewing a full paper by mid-March.
If they invite a full paper, you send it in by late April, and they send a final accept/reject by the beginning of June.
Final versions of accepted papers have to be submitted by mid-July.


Their names could be a bit better, but it seems fairly clear to me that this is their intention.  If you're concerned it may be otherwise, it's always OK to write to the conference contact email to check your understanding.