The output file is specified by the -output option.
The output file is formatted in a modified QIIME classic format where the values are floating-point frequencies in the range 0.0 to 1.0.
Note that most USEARCH commands do not support OTU tables with frequencies. You can convert from frequencies to counts by using the otutab_freqs2counts command.
usearch -otutab_counts2freqs otutable_counts.txt -output otutable_freqs.txt