So far, we have only described sequentially ordered groups of elements and text nodes. Now we will see another class of patterns which describe unordered sequences and choices. Although this class of patterns has no special name in the RELAX NG specification, we will refer to them as <firstuse>compositors</firstuse> in this book. The name comes from analogy to the compositors that are defined by W3C XML Schema. The W3C's use of the term compositors matches our usage: the name describes patterns which are composed of less complex patterns, including the basic patterns we've been looking at such as element, attribute, and text.

One of the key differentiations between compositors and simple patterns is that compositors are patterns which do not directly map to any individual element within the schema. I emphasize that compositors don't directly map to an element since it can be easy to forget this distinction when we focus on a schema instead of the instance document.