Hello there.
I want to modify the mesh by using some sop node, but I only want to modify part of it.

When I use ZBrush, I can use mask and polygroup, and limit my modification only to visible part.

Can I do the same in houdini ?

For example :
I use houdini Clip node to cut my mesh (human body), but I only want to trim some parts (fingers),
the clip trim away everything on the plane (including the legs), this is not what I want.

In ZBrush, I can mask the fingers, change to polygroup, and then limit the ClipCurve only cut the fingers.

Sorry for my bad english.

You want groups. They control which elements on which an operation is applied. You don't always have to formally create a named group to limit an operation in scope. Many operations will take the current selection as the group input.

http://www.sidefx.com/docs/houdini/model/groups.html [www.sidefx.com]
http://www.sidefx.com/docs/houdini/nodes/sop/group.html [www.sidefx.com]