"must appear only when"

[Dan Weinreb <dlw@exceloncorp.com>]

In line 890 (of Message Services Specification, Version 1, 11 May
2001), I found the phrase "must appear only when" to be slightly
unclear.

Does this mean "IF the-condition, THEN SequenceNumber MUST appear, but
IF NOT, we aren't saying anything about whether SequenceNumber might
appear or not" or does it mean "SequenceNumber MUST appear IF AND ONLY
IF the-condition is true" (where the-condition is "deliverySemantics
has a value of OnceAndOnlyOnce and messageOrderSemantics has a value
of Guaranteed")?