The UDH is an optional header at the start of an SMS payload that carries control information, most commonly the concatenation reference and part number of a multi-part message.
The user data header is a few bytes placed before the text inside the 140-byte payload. Its best-known use is concatenation: a reference number shared by all parts, the total number of parts and the index of this one. It is also used for port addressing and for some carrier features.
Because the header lives inside the payload, it reduces the room for text, which is where the 153 and 67 figures come from. Senders never write the UDH themselves through a modern API; the gateway builds it.
At SMSMeteor: concatenation headers are generated automatically; you send the text, the platform splits and numbers the parts.