The Case statement of the process similar to the concurrent with .. select statement.

However are you allowed to add several statements for each selection (delimited with ;)

The selection values must be constants.

Your allowed to separate more constants with the | operator.

You must assign selections for each combination of the selection signal (in this case Sbus)

You can use When others to cover the rest of combinations.

Even if you used When "11" instead of When others will you get an error in the example (how come?)
 

You will often use the Case statement to implement state machines.
 

Note - how the two "Wires" S1 and S0 are concatenated into a two-bit Sbus with the & operator.

Sin will be the integer representation of the Sbus