I haven’t checked out that specific Niagara example, but it just looks like a “Particle ID” Particle attribute.
Particle’s attributed are stored directly on the particles, the first module in the screenshot is setting it, while the second one is just fetching it. The two modules are not actually communicating, they are only storing and reading an attribute, to make it work the attribute needs to have the same name, in this case is “Leader ID”