If you have an older Scaleout and you're thinking of upgrading, here's a guide on which Scaleout Advance Model you'll need to replace your Scaleout.
If you have a SXP15 / SC2 / S15C / S15T:
Replacement Model = Scaleout Advance SA-15
If you have a SXP22 / SC3 / S22:
Replacement Model = Scaleout Advance SA-22
If you have a SXP28:
Replacement Model = Scaleout Advance SA-28
If you have a SXPLC / SLC:
Replacement Model = Scaleout Advance SA-LC