C++ Caesar Cipher encryption

任何人都可以解释这组代码如何工作吗？

string LoginAccess::decryptPass(string pass) {
int count = 0;
while (count < pass.length()) {
    if (isalpha(pass[count])) {
        //For Caps lock 
        if (pass[count] > 64 && pass[count] < 91) {
            if (pass[count] < 88) {
                pass[count] += 3;
            } else if (pass[count] == 88) {
                pass[count] = 'A';
            } else if (pass[count] == 89) {
                pass[count] = 'B';
            } else
                pass[count] = 'C';
            //For small char
        } else {
            if (pass[count] < 120) {
                pass[count] += 3;
            } else if (pass[count] == 120) {
                pass[count] = 'a';
            } else if (pass[count] == 121) {
                pass[count] = 'b';
            } else
                pass[count] = 'c';
        }
    }
    count++;
  }
return pass;
}

64、91等的数字是什么意思？为什么只设置A，B，C？其余的字母会发生什么？