According to the WIPO Standard ST.26 [wipo.int] :
"The symbol “t” will be construed as thymine in DNA and uracil in RNA. Uracil in DNA or thymine in RNA is considered a modified nucleotide and must be further described in the feature table as provided by paragraph 19." - see paragraph 14.
So it is necessary for a user to transform each 'u' symbol to a 't' in an RNA sequence in order that it is compliant with WIPO ST.26. WIPO Sequence will perform this task automatically during an ST.25 import and if the RNA sequence which contains a 'u' symbol is manually created within a project, WIPO Sequence will not allow you to continue with the save. Users are also able to cut-and-paste sequences into the project after replacing the 'u' symbols using a text editor.