SMS Volume Determination

During the Campaign setup process we stopped at this question not because of the number, but because of the 6 messages per minute threshold.
Does this apply to both inbound and outbound?
If a number exceeds the threshold, will they roll over? In other words, if I exceed the 6 messages in a minute threshold, will the seventh simply be delayed?
We ran a report and found there were a total of 11 instances in all of last year where we exceeded that threshold. The difference between $18 and $120 is significant for us.
Much thanks in advance.