Quick Explanation

The USD documentation does a good job of explaining this concept. Essentially, plugin metadata is a way to add custom metadata information onto prims, attributes, layers, etc. that you wouldn't normally be able to add to. It's also easy to set up. The steps can be boiled down to:

1. Create a plugInfo.json file that describes the metadata that you want to add
2. Include the plugInfo.json parent directory into the PXR_PLUGINPATH_NAME environment variable so that it is discoverable by USD
3. Work as you normally do. Get/Set the metadata.

This folder shows how to query the metadata, get information about it it in USD's schema registry, and how to author the plugin metadata plugInfo.json file.

How To Check If Plugin Metadata Loaded

C++

This gets you fallback information for metadata that is meant to be authored onto a SdfPrimSpec.

auto fallback = pxr::SdfSchema::GetInstance().GetFallback(pxr::TfToken {"my_custom_double"});
assert(fallback == 12.0 && "Plugin Metadata was not sourced correctly");

This gets you fallback information for metadata that is meant to be authored onto a SdfLayer.

assert layer.pseudoRoot.GetFallbackForInfo("another_metadata") == Vt.DoubleArray(
	[5, 13]
), message

See Also

https://graphics.pixar.com/usd/docs/api/sdf_page_front.html#sdf_plugin_metadata