As you know the USLE deals with surface erosion. Lets just visualize you were dealing with bare soil, so the cover value would be very low unless perhaps there was an organic surface layer that should influence the K factor or the C factor. So to prevent erosion and hold seed to the land, you might add mulch, either hydro mulch, straw or other formulation such as PAM. These activities to reduce erosion or promote infiltration would be addressed in the P factor, as they are practices designed to reduce erosion or promote infiltration. Things can get complex, such as when adding agricultural terraces, that do not necessarily effect K, C or P as far as erosion, but do interrupt SL factors and concentrate flow, for which is not addressed in USLE. These are just examples I made up, so hopefully there is nothing that would lead you astray. The P factor is often considered to be 1, but it is a way to adjust erosion when one or more practices are specifically directed to reduce erosion, and not adjusted somewhere else such as in the C factor. You might look into the WEPP (Water Erosion Prediction Project) analysis approach to address more specifics, detail and complexity on erosion and sediment delivery.

To put it in simple mathematical terms:

a factor of P = 1 doesn´t change the rest of your equation (as everything that you multiply by 1), giving you a value for soil erosion in tonnes per hectare for whatever parameters you chose for the rest of the equation. A factor of P < 1 (actually from 0 to 1 excluding 0 and 1 itself) reduces the calculated soil erosion reflecting on practices of reducing soil erosion and resulting in a lower value for soil erosion in tonnes per hectare.

P =1 in case of no data is acceptable from my view.