Does LiveAddress check for apartment number and suite and those types of data?
To better understand the secondary number, let's look at the primary number first. The primary or "house" number is validated and confirmed 100% to be a real, physical location. The main reason for this is that the USPS wants to know if the dwelling or structure exists.
The secondary number, according to USPS standards, has less-strict validation. Ultimately, the USPS is in charge of getting the mail to the destination delivery point so they have rules and processes that govern how to get the letter or parcel to its intended recipient even when a unit number is on the printed address but doesn't actually exist. They want the mail to get to where it's supposed to go.
At the same time, there is a level of validation that is performed on the unit number. First of all, when a printed address shouldn't contain a unit number but does, there is a "DPV code" that we return that you can inspect. This DPV code (also known as a delivery point verification code) tells you that the real address doesn't contain any secondary information even if the input address does. At that point, the secondary information becomes "extra information" that helps the postal worker delivering the package to get it to its intended recipient. Furthermore, if the address should contain secondary information, but doesn't, we return a different DPV code. Yet even in this situation the postal worker is able to correctly deliver the letter/parcel. This may be because it is a multi-unit/apartment building contains a base or "default" address which is common to all units in the structure. Think of it as a kind of "catch-all" just in case people get the apartment information incorrect.
We also run checks to see if the apartment information is within a valid range. For example, if the building contains apartments 1-9 and you supply "752", we catch that. Furthermore, if the apartment information should contain a letter, such as #12B and you only supply "12", we'll catch that as well.
One other thing to note is if the tenant of the unit is a business, we employ additional validation mechanisms above and beyond what is described here to verify the unit number.
All of the above rules are enforced according to rigid USPS standards. Ultimately the USPS is the one delivering the mail so they are the ones who know best what to do when the unit information is a little quirky.