previously, what I've asked is [here](https://graphicdesign.stackexchange.com/questions/143924/about-transparent-pixels-layers-in-photoshop). now, if it is assumed that there are no pixels and I am selecting no pixels but an area, why "lock transparent pixels" command from the layers panel for the selected layer, prevents painting the layer with brush tool etc. inside as well as outside the selection?

after filling the selected area with color the "image>trim" command trims the transparent pixels around it but if the selection is not filled then the output of the command doesn't show any difference.

another question arises after all this consideration : what is the difference among "empty area (in my words "empty pixels")","transparent pixels", "filled pixels"?