In the context of a performance resource the attribute adlib marks a resource as optional.
To indicate the tuning of an instrument, the attribute trans.diat can be used.
<elementSpec ident="perfRes" module="MEI.header">
<gloss xml:lang="en">performance resource</gloss>
<desc xml:lang="en">Name of an instrument on which a performer plays, a performer's
voice range, or a standard performing ensemble designation.</desc>
<classes>
<memberOf key="att.common"/>
<memberOf key="att.authorized"/>
<memberOf key="att.bibl"/>
<memberOf key="att.edit"/>
<memberOf key="att.lang"/>
<memberOf key="att.perfRes"/>
</classes>
<content>
<rng:zeroOrMore>
<rng:choice>
<rng:text/>
<rng:ref name="model.textPhraseLike.limited"/>
<rng:ref name="perfRes"/>
</rng:choice>
</rng:zeroOrMore>
</content>
<remarks xml:lang="en">
<p>In the context of a performance resource the attribute <att>adlib</att> marks a resource as optional.</p>
</remarks>
<remarks xml:lang="en">
<p>To indicate the tuning of an instrument, the attribute <att>trans.diat</att> can be used.</p>
</remarks>
</elementSpec>