Any occurrences of $U in the template are replaced with the username (local part) from the original address. Note that usernames of the form a."b" will be replaced by "a.b" as current Internet standardization work is deprecating the former syntax from RFC 822 and it is expected that the latter usage will become mandatory in future.
Any occurrences of $0U in the template are replaced with the username
from the original address, minus any subaddress (and subaddress
indication character such as
+). Any occurrences of $1U in
the template are replaced with the subaddress and subaddress indication
character, if any, from the original address. (See Section 188.8.131.52 and
Section 184.108.40.206 for background on subaddresses.) So note that $0U and $1U
are complementary pieces of the username, with $0U$1U being equivalent
to a simple $U.
$0U and $1U are most commonly used in PMDF MessageStore rewrite rules, where it is common to force the account portion of the local part to lowercase while retaining original case in the subaddress since the subaddress in a PMDF MessageStore address indicates a folder name. For instance, a rewrite rule: