In the type properties panel I see there is a new entry in “Create Parameters” / “By Type”:
Data

I imagine this is for “Blind Data”?
How is this new “Data” retrieved and accessed?

No this is to add parameters to a node - the type in this case being the various parameter types listed in the left hand panel. To create attributes (Houdini's term for blind data) use an attributecreate node, or an attributewrangle node.

Yes, and now in the parameter interface there is a “Data” parameter that can be added. Yet there are no options to set anything in the interface once added. This appears to be new in H15.

Attached is a screen-shot of what I'm referring to.

I'm wondering if this parameter is:
- for storing data which is “blind” (or unknown/unrecognized by houdini, yet useful for cooperating with 3rd party libraries etc),
- if there are python or hdk commands to set and retrieve such data
- and what the motivation and use case for this “Data” parameter could be

you can store and retrieve geometry to that parameter

so if dataparm is your Data Parameter
and sopnode is some sop node containing geometry you want to store in the Data Paremeter you can do

dataparm.set(sopnode.geometry())
then at any point to get that geometry you do
dataparm.eval()
or
dataparm.evalAsGeometry()

EDIT: and the motivation is probably to provide fast and flexible way to cache arbitrary data (well, at least geometry) for your tools in order to avoid slow and dangerous workaround using editable nodes and node locking within HDAs

I understand now, thanks.

tamte
and the motivation is probably to provide fast and flexible way to cache arbitrary data (well, at least geometry) for your tools in order to avoid slow and dangerous workaround using editable nodes and node locking within HDAs

Yes, one of the main goals for the Data Parameter is to provide a better alternative to editable nodes in an HDA. You can see an example of it in use in the Curve Groom SOP. In that HDA, the “Cache Strokes” button (an invisible parameter) is used to store some geometry in a data parameter and a Python SOP in the HDA makes use of the stored geometry.