The R-Series section of FreePBX HA is for setting up and managing your Digium R-Series DAHDI Appliances. Below is an outline on how to set up the device.
We can see that the R-Series is currently sending calls to the primary server and it is in T1 mode. Pressing the Switch button will force the R-Series to send calls to the backup server. (This is not something you would ever really do unless it was out of sync, since the backup server won't have Asterisk running.)
If at some point you migrate from PRI to pure SIP/PJSIP/IAX2 trunks you will need to remove the service before removing the R-Series or the cluster will crash. Use these commands