JSON-LD
JSON-LD is a way of embedding structured data in a page as a single block of JSON, rather than as attributes scattered through the markup, and it is the usual format for publishing product facts an agent can read.
The practical advantage over attribute based formats is that the data is not tied to the layout. A redesign moves elements around and the block survives, where markup attributes are frequently lost the first time a template is rewritten.
What matters more than the format is where the block is. One emitted in the served markup is readable by anything that fetched the page; one injected by a script after load exists only for something that rendered the page, which is the more expensive path and often not taken.
Why it matters to a store
A JSON-LD block that validates and omits price or availability is a common and quiet failure: it passes every checker and answers none of the questions a comparison asks.