The bit indicates that its associated block of memory has been modified and has not been saved to storage yet.
Dirty bits are used by the CPU cache and in the page replacement algorithms of an operating system.
This technique can be used with delayed computing to avoid unnecessary processing of objects or states that have not changed.
When the model is updated (usually by multiple sources), only the segments that need to be reprocessed will be marked dirty.
Afterwards, an algorithm will scan the model for dirty segments and process them, marking them as clean.