Light mode is used mostly by the monerod, validating incoming blocks. But on a machine with sufficient free RAM, monerod can also be set to use Fast mode instead.

The post described both modes. The only difference is that Fast mode processes the cache to generate the full 2.1GB dataset, so subsequent programs can just reference it as needed. Light mode uses only the 256MB cache and generates the required dataset values individually, on each access. That saves RAM but costs more CPU time.