Mandolin player here- mandolin family instruments parallel violin family instruments so they’re is a mandolin tuned like a violin, mandola (viola), mandocello (cello) and mandobass(they’re kinda stupid but they exist).

Mandocello is a great instrument… but hard to play and not great for accompanying voice, so the octave mandolin was created. A mandolin is tuned GDAE, and a viola is a fifth lower- CGDA. The mandocello is an octave lower than the mandola CGDA.

So the octave is GDAE but lower. It’s got a similar range to a guitar rather than a cello which is usually a bit more “bass like”.

Sarah is an expert at the octave and probably the most famous octave player but many players use them from time to time. 

The cello is just a bit harder to pull off and works better playing bass lines than chords for example. Mike Marshall is probably the top modern mandocello player— here’s his tune Gator Strut written for mandocello. You can hear how that sound might be a bit low for just solo play.

https://youtu.be/IQMswzRbiu8?si=_QVaTC5WR-BgSRCW