VOP For Loop usage

The inputs of the VOP For Loop are a bit annoying.
Length - this is the number of loops to be made. It is the i<10 part of the loop.
Index - you don't have to set this unless you need to start with a value other than 0. eg, you want to start the loop with for(i=3;i<............)
The rest is just for any attributes/values you need to use.

Attribute Interpolating UVs & things other than scattered points.

Creating "sourceprim" and "sourceprimuv" attributes for things other than scattered points. In the Houdini docs for the scatter node, there is a guide on getting your scattered points to follow animated/deforming geometry. The scatter node can generate the source primitive and it's uv attributes for you at the tick of a box. You then stick that into an attribute interpolate node and away you go.

However, if you want to interpolate something else like say..a colour attribute or a UV attribute, or something like that - then you'll have to create your own sourceprim and sourceprimuv.
Freeze your geo using a timeshift. Then just lay down a pointVOP, connect it to your geo & then plug the source (ie in the case of the scatter example, it's the object you want to stick to) into the second input.
Within the VOP make an XYZdist node and wire up P to the position input and the second input into the "file" . Create two bind exports and change the output names to sourceprim and sourceprimuv, making sure the data types are correct.
Finally just play around with the search distance in the XYZdist node so you get correct values. Jump out of the VOP and plug the appropriate bits into the Attribute Interpolate node. Adjust the settings to whatever you need (Vertex,point etc...) and everything should be good! You might have to change the pointVop to a vertexVop... too haha.

Checking if a point is inside a volume

This can be done with a point wrangle, but for those that prefer VOPs...

create a point VOP, connect the geometry you wish to test into the first input. connect the volume you want to test with into the second.

Enter the VOP, create a volumesample. Connect P and OpInput2 into this node.
The value it returns (bind export this to something) indicates how "deep" within the volume the point is. A negative value means the point is within the volume & a positive value means it is outside of the volume.