Understanding a function that reads data from a serial port - c++

Question

I'm struggling to understand what is happening in part of an Arduino program I wrote, I copied the switch in processInput from another post about reading data from serial ports but have changed it a bit since then.
To me it seems like the output to the LCD screen should always be 0 since recievedNumber is set to 0 at the beginning of the function, it does output the data from the serial port correctly though. I'm also not sure what is happening on the line "receivedNumber += b - '0';" inside the last case.
Any advice or a breakdown of the logic would be appreciated, I intended to replace the last case with an if statement to learn how to do it without using the GNU extension but I just don't understand what is happening.
An example of data in the serial port:
<45,56><55,54>
Sorry if this is the wrong place but it felt too related to programming to post on the Arduino StackExchage.
const char startOfNumberDelimiter = '<';
const char middleOfNumbersDelimiter = ',';
const char endOfNumberDelimiter = '>';
void processNumberC (const long c)
{
lcd.setCursor(10,0);
lcd.print(c);
lcd.print("C");
}
void processNumberG (const long g)
{
lcd.setCursor(10,1);
lcd.print(g);
lcd.print("C");
}
void processInput ()
{
static long receivedNumber = 0;
byte b = Serial.read ();
switch (b)
{
case middleOfNumbersDelimiter:
processNumberC (receivedNumber);
receivedNumber = 0;
break;
case endOfNumberDelimiter:
processNumberG (receivedNumber);
break;
case startOfNumberDelimiter:
receivedNumber = 0;
break;
case '0' ... '9':
receivedNumber *= 10;
receivedNumber += b - '0';
break;
} // end of switch
} // end of processInput
void loop()
{
while (Serial.available ())
processInput ();
}