Usually the + means more Republicans than Democrats (as in the FL numbers).
Sometimes it means an increase over 2012 numbers so you’d have to look at context.
But normally if someone says, when discussing absentee ballots, Rs +30,000, that means so far the Republicans have 30,000 more absentee requests than Democrats.
As to your second question, absolutely no insight as to how a pollster would describe these people. Actually, I doubt they’d ever ask “when did you register?” They would likely say, “are you a registered Democrat or Republican or independent?”
Surely the pollster would ask that question to the person being polled. But if the poll was designed to include only LIKELY VOTERS, they would have to ask a few screening questions to classify the interviewee as likely or not likely. Just wondering whether a newly registered voter would fail that "likely" test, simply because he or she doesn't have an established record of voting in the past.