In addition to categorizing UPS systems by how they work, they can also be further subdivided by output capacity, configuration, intelligent operation, and extended run capability. UPS systems can be ranked by the maximum amount of power, expressed in both VA and watts, that equipment is able draw from the UPS outlets. "VA" stands for "volt-amps." The VA value of a device can be determined by multiplying its operating voltage by its amperage rating. For example, a 120 V, 2.5 amp device would draw 300 VA from a UPS outlet. UPS systems can also be subdivided by the type of cabinets they are housed in such as rackmount, tower, low-profile, undermonitor, etc. Within the line-interactive category, there are two subcategories—those that feature intelligent operation and those that do not. UPS systems that feature intelligent operation include a microprocessor that allows the UPS to relay more detailed operating conditions through PowerAlert power management software. All on-line UPS systems feature intelligent operation. Finally, within the line-interactive and on-line categories, there are two subcategories—those that accept connection of additional internal or external batteries and those that do not. Those that accept additional batteries allow users to extend battery backup runtime for critical systems during a blackout which is called Extended Run Capability.