- of the environment variable split up
- at whitespace, resulting in zero or
- more arguments. To pass a literal dollar sign,
- use <literal>$$</literal>. Note that the first
- argument (i.e. the program to execute)
- may not be a variable.</para>
+ of the environment variable split at
+ whitespace, resulting in zero or more
+ arguments. To pass a literal dollar
+ sign, use <literal>$$</literal>.
+ Variables whose value is not known at
+ expansion time are treated as empty
+ strings. Note that the first argument
+ (i.e. the program to execute) may not
+ be a variable.</para>
+
+ <para>Variables to be used in this
+ fashion may be defined through
+ <varname>Environment=</varname> and
+ <varname>EnvironmentFile=</varname>.
+ In addition, variables listed in the
+ section "Environment variables in
+ spawned processes" in
+ <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ which are considered "static
+ configuration", may be used (this includes
+ e.g. <varname>$USER</varname>, but not
+ <varname>$TERM</varname>).</para>