It's an EID counter. Each 16 byte EID number comes from an external reader. When you have two rfid's hanging around the antenna without moving past (sheep in my case), the reader has a habit of reading and sending the id's alternately as fast as it can (currently at 9600 baud.) When my mega receives each EID it has to check each EID number against a list of around 1,000 others. Using binary search I cut down the search time to between 7ms and 78ms per id, depending on where in the list it appears. When the EID is found there are five fields taken from a reference file and attached to that EID and written to a log file. All this for each and every EID, and this while the reader (sometimes) pumps out the numbers at pace.
With the buffer at 64 bytes it only takes four EID numbers to fill the buffer and make my mega go nuts. After I adjusted the buffer up to 256 bytes yesterday she eats up the numbers like a hungry lion. No matter how fast I throw the EID tags at it, it digests the data and patiently wait for more.