By default, an ukulele is tuned in reentrant tuning, i.e., the 4th string does not sound lower than the 3rd, but a fifth higher. The lowest sound is therefore on the 3rd string.

To extend the range (possibly also for reasons of aversion to the reentrant tuning), the 4th string can be replaced by a special string that sounds one octave lower than usual. This is no longer a fifth, but a fourth below the 3rd string.

To distinguish between reentrant and linear tunings, this is called, in the case of the universally most widespread C6 tuning, high G for reentrant and low G for linear (non-reentrant) instruments. (In the case of the D6 tuning, this would be high A for reentrant and low A accordingly).

However, soprano ukuleles are considered to be less suitable for the linear tuning. James Hill states:

Low-4th tuning doesn't work on soprano-sized instruments unless a wound 4th string is used … The nylon low-4th string works in D6 tuning because the tension on the 4th string is higher.1)
Linear C6 tuning

g-c'-e'-a' (G3-C4-E4-A4)


Glen Rose: Why I use a Low G on my Ukulele (2019)

James Hill: On the Road with ArtsCan Circle. Ukulele Yes! 9:1 (2010)