Well Good X Practices (GxP) is an FDA regulation covering the manufacturing and distribution (among a lot of other things that's where the x is). Reefers are to be tested under Good Manufacturing Practices (GMP) and Good Distribution Practices (GDP) to ensure that things don't go bad during the chain of custody (aka transportation).
There is another standard, National Institute of Standards and Technology (NIST), which also covers a number of things that are used in Manufacturing and things like Art and high value things.
Then there is Hazard Analysis Critical Control Point (HACCP), which is used in food manufacturing - many of us don't deal with this. I did for a bit but it was a pain in the a**. I hated washing out my box so I can haul three pallets of food dye. It also covers food safety as in food handling at those fantastically clean truck stops.
Basically the box, recorder and reefer are all tested to ensure that it ramps up (or down) temps, hold temps and the probes work properly, print outs are accurate and the temp tolerance is held under most conditions. While that is done, there is test results documentation which is part of the chain of documentation that is created (a lot like the quality standards under ISO what ever) and those are kept on record with the carrier so they can be sent to the customer for their compliance requirements on some products.
The last time I had this one validated, it cost $350 plus 24 hours of fuel, not to mention the hotel bill and losing two loads. It failed and hence don't run a validated reefer. The problem had nothing to do with the unit but the qualcomm and how it was tied into the recorder/reefer unit so I decided that it wasn't worth the effort to change it unless I did more than three runs a month, I've done three in two years.
By the way, be aware of sticker shock on labor for reefer units. I work on my own, it is simple for me to do my own work, I have all the tools now and Carrier provided all the manuals for nothing but nevertheless the last time I had this thing worked on the bill came to $3,800. The time before that is was a $1200 injection and fuel pump. I know one guy who runs for FedEx dumping $7000 into his unit, rebuilding the engine's head and replacing the compressor among a few other things and that was with a hefty discount. I can make a good point that one or two real issues with the unit will wipe out your profits.
Also as I am thinking about it and sorry about forgetting this, the age of the truck isn't a real issue but the box age is. The insulation breaks down. Depending on how well the box was constructed, they may not start until the 5th year. However, I have seen a couple that were made like crap and the reefer unit had a hard time holding temps and that was caused by the insulation already breaking down.
My question as I'm thinking about it is have you had reefer experience?