Deratting cert. is proof that you have had someone to check you have no rats onboard.
We cruised through the Maldives
2003, I can't remember if I had to sign that I had no rats, but I certainly did not have to have an inpection for rats, nor did I anywhere in the world on a 10 year circumnaviagtion. You do however have to sign some wierd forms sometimes. EG. No stowaways, no deaths, no plague. We did get inspected on leaving Massawa to make sure we were not helping the locals to leave. I also had to make a list of what I DID NOT HAVE onboard, as opposed to what I did! Could have taken a long time but after about 10 items the customs
guy was happy.
The normal rule
for yachts these days is to clear in and just follow the rules as presented on the day, they change not only often, but sometimes from yacht to yacht on the same day.