The group field at the top of most sops will give you a dropdown of group names you've defined upstream.
You can also use this field to define group expressions. Say you want to delete all the points below 0 in Y. Put down a delete sop, and use the group
@P.y<0
What's powerful about this is you don't need a group node at all; in any sop that has a group field, you can use that expression where you'd normally type the group name, and it'll work.
Make sure you don't have spaces in the expression though! Group fields let you specify multiple groups like 'group1 group2 group3', so if you use the expression
@P.y < 0
Houdini parses that into 3 groups, one called '@P.y', another called '<', and one called '0', which of course won't work. If you need spaces (say you're matching against multiple point id's), enclose it in double quotes.
This is often used with a blast or delete node, eg delete all points that have full red in their colour ( @Cd.x==1 ), or their id is 5 or 10 or 23 ( @id="5 10 23" ). The help for this is kinda buried in the docs here:
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