[Question] Storing custom data properties on ElkNode and ElkEdge objects (to be used later during rendering)?

[IarwainBen-adar]

Is there any documentation available on storing custom properties on ElkNode and ElkEdge objects?

From examining the ELK source and metadata descriptions, and from trial and error within the ELK-Live json-to-elkt converter, I have come to the initial conclusion that custom data property attributes are allowed to hang off the base definition of an ElkNode or ElkEdge object, as in:

{
    id: "n1",
    tokenData: {
        category: "node",
    }
}

But if I try to store custom data properties inside the properties block itself, ELK throws errors:

{
    id: "n1",
    properties: {
        tokenData: {
            category: "node",
        }
    }
}

Is the properties block itself a special case for ELK? I can find very little (if any) mention of the properties block in the ELK API documentation.