It's just a separator. The syntax of the command is:
{x>y:dest}
x is usually the CallerID
y is usually the UserID
{>17771234567:ph} is checking if the call is coming from your Callcentric account.
{800555121>17771234567:ph} is checking if the call is coming from your Callcentric account and the CallerID is 8005551212.
The > is not required if only checking for CallerID.