The copula in predicative sentences can be null. For example, in Russian, the sentence <i>Ona vraÄ</i> 'she doctor' the finite form of <i>be</i> is obligatorily null (Stassen 2008). In standard English, on the other hand, the copula is obligatory (<i>she *(is) a doctor</i>). A 'yes' value for this property indicates that the language allows null copular constructions, as in Russian. 
<br><br><b>Reference</b><br>
Stassen, Leon. 2008. Zero Copula for Predicate Nominals.
In Haspelmath et al. (2008), chapter 120.
Available online at <a href="http://wals.info/feature/120">http://wals.info/feature/120</a href>.