Tag Expression Basics
Tags form a fundamental building block upon which much of HL is constructed, and tag expressions are where they become of critical importance. Since the vast majority of objects you'll be managing are things and picks, there must be a way to identify the proper subset of these objects that apply to a particular situation. For example, attributes, skills, and weapons are used in completely different ways, so you want to keep them separate from each other – yet they are all things (or picks). The solution is to assign tags to each object and then use a tag expression (or tagexpr for short) to identify the subset of objects that apply in a given situation. A major (separate) section of the documentation is dedicated to the subject of tag expressions, but a brief overview is valuable at this point.
A tag expression is essentially a filter that gets applied to all objects of particular type (e.g. things or picks) and selects only the ones that meet the specified criteria. Tag expressions are Boolean expressions, which means they evaluate to a simple "true" or "false" result. They examine all of the assigned tags and determine whether those tags satisfy the expression or not. Separate criteria can be combined, allowing you to require that multiple criteria all be met, one of a set of criteria be met, certain criteria be excluded, or some combination thereof. For example, a tag expression could test whether a thing has the tag "Elven" from the "Language" group. Or a more complex tag expression could test whether a thing has the "Language.Elven" tag and also has either the "Race.Elf" or "Race.HalfElf" tag.
Since tag expressions can utilize full Boolean logic (i.e. "and", "or", "xor", "not") and can even extract and test numeric values from tags, tag expressions can model extremely complex conditions without difficulty. The bottom line with tag expressions is that they provide a powerful and flexible method for quickly determining whether to include or exclude an object, and they are based exclusively on the set of tags assigned to that object. As such, they are used extensively throughout HL.