This is the name of a log measured along a well bore (a well-log). You could have found the code
PO8C in your log evaluation software or your corporate datastores.
Partial porosity from MRIL (single element) is the offical name for this log. This log is often used in the industry. This log displays readings of the contribution of porosity from a given "bin" (with certain characteristics) to the total porosity; e.g., in nuclear magnetic resonance data processing, the "bins" typically correspond to different pore size ranges. In general this log therefore displays units of the apparent porosity. Better said, the initial estimate of porosity, normally from logs, not corrected for the (often large) effects of clays, gas, etc. Such estimates should only be used for qualitative interpretation of clean, gas-free intervals.
Concluding, this is a measure of the pore volume per unit gross volume. Porosity is determined from measurements on cores or interpreted from logs. At a more general level these values are measures of the volume fraction. This can be explained as being the ratio of the volumes occupied by two components, or by one component to the total volume of the system.
Data for this log is produced by a tool that uses a nmr logging technique and can be used to investigate the porosity of the formation. Similar logs are listed below: