Generally speaking, partitioning an alarm system will allow you to have 2 or more independently controllable areas, as well as limit or control access to either by use of codes, and sometimes, code authority levels.
Partitioning a system can be beneficial if you have outbuildings that you would like to be secured independently of a main area, as well as have them remain armed or disarmed while other areas have their states changed. One can also assign partitions within a single house, such as if a gun cabinet or safe is connected to the alarm and it is desired to be armed the majority of the time, irregardless of if the house is armed or disarmed.
Generally it is a recommended procedure to have a keypad installed in, or for, each partition a system has, both for knowing system status and ease of use. Some systems will allow access to multiple partitions from a single keypad, which is handy if you have outbuildings and need to arm or disarm from within the house. Also take note of how a system's siren or bell output will react in the case of multiple partitions and alarm conditions.
Usually, codes on a partitionable system have some general requirements and access considerations. Most systems will have a system master code, as well as a partition master code. A system master code usually has the ability to access the entire system and partitions, while a partition master code is limited to the partition, or any other partitions they are assigned as a user. Code attributes and access abilities can really get complex, such as a partition master having no or limited access to other areas. Generally all systems will allow common users on partitions as well.
Of interest something which is referred to as a "common lobby" or "affects other area" partition. Generally, these aren't true partitions, but used to secure an area which connects 2 partitions. They will usually follow the states of the connected partitions, with a "first in, disarm" and "last out, arm" logic. Usually the application and use of these "partitions" would be limited and only used in a commercial application.