I can already hear you ask "What's the importance of tags?" so let me briefly explain.
The tag on a Moto radio is one of the last things that is put on it before it leaves the Moto factory. This contains information about the radio such as the model number, flashcode, and most importantly, the serial number. The serial number is important because it determines whether the radio is a legitimate and/or if it has passed the inspection of Motorola. It's not uncommon for an APX radio to be an ALT (Accelerated Life Testing) radio which means Motorola performed their spec testing on that radio (waterproof testing, shockproof testing, etc). A legit tag also means that the radio is not a "parts built" meaning that someone got all the parts they needed to build the radio and Frankensteined it together. Since tags contain the serial number, sometimes people that have a stolen radio will scrape the tag off in order for the end user to not be aware that the radio is stolen.
Lots of flags come up with radios that have no legitimate tag. Depending on your use for it, it may or may not be a big deal. Either way, if you are buying an APX, make sure it has a tag and buy the seller not the radio.