These are punctuators and %: is the same as # and %> is the same as } etc... The C99 draft standard section 6.4.6 Punctuators says:
In all aspects of the language, the six tokens67)
<: :> <% %> %: %:%:
behave, respectively, the same as the six tokens
[ ] { } # ##
They are also sometimes refereed to as digraphs as we see in note 67:
These tokens are sometimes called ‘‘digraphs’’.
