Primary gatekeeper switchback:
In case that the primary gatekeeper stops responding, the equipment could switch to a secondary one.
Once it is working via the sec. gk, it does have the option of switching back to the original or staying on the secondary.
That is what the setting is about.
Packet saver: If you have the option on your box, and the remote box also have the option, and you have several calls at the same time betwen the two gateways with packet saver enabled, you will save some bandwith on the interent connection.
remember, omly if you have several calls at the same time ,betwen the two boxes.
It will not give you any advantage on single calls to multiple boxes, even if they have packet saver.
The savings are around 15 to 30 %,depending on the load.
Best case will be fully load of calls, point ot point betwen the boxes.