The following document provides links to helpful resources relating to FASTA files from the UniProt knowledge base.
Here is the downloads page for the UniProt knowledge base. Click on the FTP link to find the specific FASTA file of interest.
Here, general information on the FASTA format can be found.
This link to the NCBI provides more information on the FASTA file format.
This page from UniProt explains how to decode FASTA headers (i.e. what does the SV number or PE number mean).