When we redistribute something into an OSPF Not So Stubby Area (NSSA), then these prefixes are flooded within the NSSA area as OSPF LSA type 7. Once these LSAs make it to an ABR, they are translated into LSA type 5 and advertised to other areas.

The P-bit (P stands for propagate) can be found in the options field of an LSA type 7 and it tells the ABR if the LSA type 7 should be translated into a LSA type 5 or not. Only LSAs with the P-bit will be translated and automatically this bit will be set for all prefixes that are redistributed.

In IOS versions before 15.0(1)M, there is no way to directly enable or disable the P-bit. This must be done using either the OSPF not-advertise feature or the OSPF Forward Address Filtering feature.

However, in IOS versions 15.0(1)M and later, the nssa-only keyword was added. This keyword is able to manually set the P bit to zero without the need to do so using the above mentioned options.

Take a look at OSPF - summary-address nssa-only on an ABR for more information on a similar command and outcome.