This is a well log used in the oil and gas industry. You could have found the code
PO7C in your log evaluation software or your corporate datastores. The official name for this log is
'Partial porosity from MRIL (single element)'. This log is often used in the industry. This log can be used to evaluate 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 thus reflects 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.
So basically this is a measure of the pore volume per unit gross volume. Porosity is determined from measurements on cores or interpreted from logs. In general this is a measurement of the volume fraction. In other words, 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: