The QBLAST system located on the BLAST server executes the search, writing information about the sequence alignment in ASN.1. The results can then be formatted by fetching the ASN.1 (fetch ASN.1) and fetching the sequences (fetch sequence) from the BLAST databases. Because the execution of the search algorithm is decoupled from the formatting, the results can be delivered in a variety of formats without re-running the search.
From: Chapter 16, The BLAST Sequence Analysis Tool
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