allocation provides a snapshot of how many shards are allocated to each data node and how much disk space they are using.

GET /_cat/allocation?v

Might respond with:

shards disk.indices disk.used disk.avail disk.total disk.percent host      ip        node
     1         260b    47.3gb     43.4gb    100.7gb           46 127.0.0.1 127.0.0.1 CSUXak2

Here we can see that the single shard created has been allocated to the single node available.