Originally Posted by pinseeker

Because, if you drop the trip, you get paid 150% for the first 12 hours of extra pay and 200% for any hours after that. If you elect substitution, you get 100% pay for all hours that the original trip extends into the next trip(s). It all depends on how long the original trip gets extended, hence the suggestion to wait until you block in to make the final decision so you aren't leaving money on the table.