Speed rating. The tyre must be rated to at least the vehicle's maximum speed, fairly obvious! It's a measure of the tyre's ability to withstand the centrifugal forces associated with increasing speed. Higher is better.It just says to use seat approved tyres and doesn't state sizes or ratings
V 149 mph
Z 150+
W 168
Y 186
So V rated or above will be fine other than on a Cupra, although most are supplied with W or Y rated tyres.
Load Rating.
The maximum load for each tyre.
91 = 615kg
94 = 670kg
91 is common on the current VAG hatchbacks and coupes. Estates may well be specced to 94 to cater for the extra load they may experience.
The higher rating comes from using a stiffer sidewall so use an appropriate rating for your vehicle, adding a higher rated tyre may reduce tyre deformation and adversely affect braking/handling performance.
In short use a 91 Z, W or Y rated tyre unless you have an estate. HTH