USEARCH allows lines of any length in a FASTA file. (Some programs limit lines to e.g. 80 characters).
USEARCH does not support comments in FASTA files.
White space characters (blanks and tabs) are discarded if found in sequence data, but many other tools do not allow this and the practice is not recommended.
If the -trunclabels option is given, USEARCH will truncate sequence labels at the first white space (similar to BLAST), otherwise the full label is retained..