Almost every model of soccer cleats is made to be suitable for a different field condition.

HG (hard ground)- for playing on hard surfaces.
FG (firm ground) - for playing on firm surfaces.
SG (soft ground) - for playing on soft surfaces.

Why is it important to have the right soccer cleats for different types of field? In order to play soccer to your best abilities, you have to have soccer cleats. For several reasons but the most important is: traction. The studs will prevent you from slipping, giving you opportunity to turn, stop and accelerate with ease.

In order to achieve it, soccer cleats outsole (bottom part of the shoe) has several hard rubber, plastic or metal studs that sink into the surface providing a good grip. But the studs have to be long enough to offer good traction and short enough not to cause discomfort from applying too much pressure on the feet. In other words, the length of the studs and their count depends on types of the surface they would be the most adequate for.

It basically means that soccer cleats for HARD GROUND (surface) have short studs. They provide you with enough amount of traction but at this same time they don't apply too much pressure on your feet causing discomfort when your cleats are unable to dig into hard ground. If you play on very hard fields, Turf shoes (cleats - TF) would be your best choice.

SOFT GROUND (SG) cleats come with removable (in most cases) studs. There are few different lengths of the studs for different conditions of soft fields (soft and wet for example), so you can adjust them properly.

FIRM GROUND (FG) cleats are the most universal. FG cleats are good for all field types without going to extreme (very hard and very soft). Of course they are the most adequate for fields with the firm surface (something between hard and soft). But if you play on different types of fields and can't afford to have two or three types of soccer cleats, the FG is probably the most ultimate choice.