The makeudb_tax command creates a UDB database file from sequences in a FASTA file. The database file is compatible with the sintax, usearch_global and usearch_local commands. Using a udb index make the database faster to load, but otherwise does not change the results.
See SINTAX reference data downloads page for available reference files.
You don't need to trim sequences to the same region as your reads / OTUs.
The sequence labels in the FASTA file must contain taxonomy annotations.
usearch -makeudb_sintax rdp_16s.fa -output rdp_16s.udb