Later you snapshot /data again to create /data-2 on the source
system.
You btrfs-send /data-2 to the other system again and it creates a new
read-only subvolume - you tell btrfs-receive what to call it and
where
to put it, let's say you call it /copy-data-2 - using the data in the
stream and reusing some extents from the existing /copy-data-1.
/copy-data-2 is now a (read-only) copy of /data-2 from the source
system.

How you use that copy is up to you. If you are just taking backups
you
probably do nothing with it unless you have a problem (it will form
part
of the source for data for any future /copy-data-3). If you want to
use
it to initialize a read-write subvolume on the destination system you
can take a read-write snapshot of /copy-data-2 to create a new
subvolume
(say /my-new-data) on the destination system.

Such is close to what I have always understood about receive, but the
confusion is that the second receive command makes no reference to the
subvolume created by the first command. How do I ultimately create a
restore target that combines the original full capture with the
incremental differences?

When I ask how I use it, I mean what commands do I enter into the
system.

Note in my case I archive the streams into regular (compressed) filesm
for later recovery.