What is an alignment?

We use the term alignment to mean a directed relationship of dependency: a target location is aligned with a source location when the construction of target nodes which are target location instances requires information about the presence or content of source nodes which are source location instances. There may be a semantic equivalence between the aligned locations, but this is not necessarily the case. In our introductory example of a transformation, the target location creator which is child of a publication location may be regarded as semantically equivalent to an author location found under a book location. As a counter example, consider a target location airline whose nodes are constructed by extracting information from flight numbers provided by flightNumber elements: the target location airline depends on the source location flightNumber, but there is obviously no semantic equivalence.