In the first line above, '66677789' is the recipient's address (telephone number) and '68656C6C6F' is the content of the message, in this case the ASCII string "hello".
The final field is the checksum, calculated simply by summing all bytes in the packet (including slashes) and taking the 8 least significant bits from the result.
The two-digit transaction reference number means that an entity sending text messages can only have 100 outstanding messages (per session); this can limit performance, but only over a slow network and with incorrectly configured applications on one's SMSC (for example one session, with number of windows greater than 100).
The EMI UCP documentation does not specify a default alphabet for alphanumeric messages after decoding from hex digits.
The related ETS 300 133-3 standard specifies the GSM-7 alphabet, which accommodates more languages than ASCII by replacing unprintable control codes with additional printable characters.