Why some manufacturers express the filter's permeability as pore size while other use retention? And what the difference between pore size and nominal pore size?
For membranes, defining a pore size is easier because they have a relatively consistant shape whereas filter paper don't really have pores in the sens of capillar holes. For paper filter, the "pores" are virtual 3D paths in the matrix of tangled fibers so we just can't measure them optically. We can still attempt to aproximate an average ideal pore size based on the behavior of the filter flow. This is the nominal instead of absolute pore size.
Retention is the capacity of the filter to retain particles of a given size. To keep it simple, let's not consider particle or pore shape. For a membrane, a pore size smaller than a particle size should retains the particulate. For a paper filter, depending where the particle land, it might be retained on the surface, or in the thickness of the filter or pass through. There is a distribution of "pore" size with a probability that a certain size of particle will be retained. Moreover as the filter gets loaded with more and more particle, its flow get reduced. Retention needs to be evaluated at a given efficiency, usually > 80%.
To summarize, pore size is the ideal diameter of an imaginary tube passing through the filter while retention size is the ideal diameter of the smallest particle blocked by the filter. In filter paper, pore size measure an average distribution while retention measure a minimum and is what matter if you want to retain a given particulate.