few ways…
you can use a regular mask to select areas you want to keep. click tool > visibility > hidept to hide the rest . Then click geometry > delhidden to remove the hidden polygons.
you can use a clipping mask to display only the areas you want to see (cntrl + shift drag box) or remove specific areas ( cntrl + shift + alt) and then when you see only the pieces you want to keep geometry > delhidden
the only caveat with this method is you need to delhidden on your lowest subdiv level
The other remaining option which lets you keep the rest of the mesh in case you decide later to go back and reattach is using clipping mask again, make sure to make only what you want to see visible, click polygroups > group visible. Unhide the rest of the model. Click subtools > groups split.
That will break your model down into a subtool for every polygroup you split it into letting you control exactly what parts you have, and you can split the model at any subdivision level. The downside is if you split a model at a higher subdivision level you may inadvertantly mess up some lower subdiv level layout. If you notice this happening, before you split the tool cick geometry > crease . This will add an indentation around your polygroup edges that will help them keep their form after you split and when you subdivide again later.
best way to figure out which way works best for your needs is to practice them and play with various settings along the way.
-d