@bsoneill - because Rachio changed the terminals for rain sensors between the Gen 1 and Gen 2 and then again on the Gen 3. I believe the was a clarifying comment for someone upgrading their controller from Gen 2 to Gen 3 as how to handle it.
The link I provided in your other post actually has documentation for how to wire up a rain sensor for each of the different Rachio generations - 1, 2 and 3.