ABSOLUTE
           is for counters which get reset upon reading. This is used
           for fast counters which tend to overflow. So instead of
           reading them normally you reset them after every read to
           make sure you have a maximum time available before the next
           overflow. Another usage is for things you count like number
           of messages since the last update.

       COMPUTE
           is for storing the result of a formula applied to other
           data sources in the RRD. This data source is not supplied a
           value on update, but rather its Primary Data Points (PDPs)
           are computed from the PDPs of the data sources according to
           the rpn-expression that defines the formula. Consolidation
           functions are then applied normally to the PDPs of the
           COMPUTE data source (that is the rpn-expression is only
           applied to generate PDPs). In database software, such data
           sets are referred to as "virtual" or "computed" columns.