A lot number is a number which is assigned to a lot of land within the context of a subdivision. Lot numbers are part of the so-called "lot and block" survey system, which is used to identify land which has been broken up into a number of smaller units. Lot numbers and assessor's parcel numbers (APNs) are both used to refer to lots of land, but the numbers used for the same piece of land will be different because different recordkeeping systems are involved.
When land is subdivided, it is broken into blocks, divided by streets and other thoroughfares, and each block is divided further into lots. The lots may be identified with numbers or letters. The lot number allows someone to identify a lot of land within a subdivision.
It is necessary to have a map of the subdivision to find the lot a specific number is referring to. The lot number does not convey additional information like the size of the lot, the zoning, and so forth; this information must be gleaned from the documentation for the individual lot. Maps of subdivisions show the outlines of the lots, with each being assigned a number. This allows people to identify lots quickly and easily — someone can say "Lot 25" instead of "the lot two houses down from the corner of Spruce and Waybrook Streets," for example. In formal documentation related to the lot, the lot number may be used as an identifier.
While land is in development, it is not uncommon to use lot numbers to refer to individual lots. Once structures are built and addresses are assigned, people may prefer to use addresses, rather than lot numbers, to refer to their property. In written descriptions of the property, the address and the lot number will both be included, along with other data to ensure that the lot is described fully and completely and to eliminate any confusion about boundaries, the precise location, and other information.
Assessor's parcel numbers also refer to lots, except that they do not use the lot number as an identifier. Instead, the APN is a code which reveals where information about that lot is recorded. Every assessor's office works differently, but the APN usually consists of a string of letters and sometimes numbers which tells people which record book the entry can be found in, and which entry is being referred to.