There are British and European Standards which define what constitutes a natural stone sett and a slab, similarly precast concrete setts and paving flags.
BS EN 1342 defines a natural stone sett as being between 50 and 300 mm in length and that the thickness should not be less than half the length.
At the larger end of this scale, we might find a sett 150 x 300 mm in plan x 150 mm thick.
It is more common in continental Europe to see cubes, so 150x150x150 mm is a common size.
At the smaller end of the scale, we commonly find a 50x50x50 mm cube sett.
Setts having a thickness equal to or greater than width are the basis for millions of square metres of streets laid over many hundreds of years, many of which survive after centuries of use. These are the basic component of Unbound Construction, where mortar can give way to simple aggregate to produce a structure which will last indefinitely.
Shallow setts are nevertheless a common feature in modern design and it can be argued that, where this is possible, a more efficient use is made of a valuable natural resource. The design requires more careful engineering and close attention to detail during construction.
BS EN 1341 defines a natural stone slab as being more than 150 mm wide and up to 900 mm in length, where the thickness is not greater than half the width. This might seem a fairly common sense description of a paving slab but an increasing number of designs ask for slabs which are longer than 900 mm, so special cases are widespread and there is much for the designer to do.
Pre-cast concrete blockss and flags, being a manufactured product, are generally compliant with the relevant Standards, BS1338 and BS1339 respectively. so we do not make a detailed comment about their characteristics here.
Paving flags and slabs, also shallow setts, can be laid without mortar in some situations but generally not where there is to be vehicular traffic.