Sometimes you need to specify rewrite rules that apply only to envelope addresses or, alternately, only to header addresses. The control sequence $E forces a rewrite to fail if the address being rewritten is not an envelope address. The control sequence $B forces a rewrite to fail if the address being rewritten is not from the message header or body. These sequences have no other effects on the rewrite and may appear anywhere in the rewrite rule template.
Addresses may also be categorized by direction. A forward-pointing address is one that originates on a To:, Cc:, Resent-to:, or other header or envelope line that refers to a destination. A backward-pointing address is something like a From:, Sender:, or Resent-From:, that refer to a source. The control sequence $F causes the rewrite to fail if the address is backward-pointing. The control sequence $R causes the rewrite to fail if the address is forward-pointing.