The level key is the mechanism to define the members of a level. It is a set of attributes and
attribute expressions. The level key must uniquely identify the member within the level. The
level key can consist of one or more attributes. The combination of these attributes will be the
level unique key.
A member is uniquely identified when the value of the level key for the member does not
match the value of the level key for any other member in the level. This uniqueness is defined
partly by the data contained in the key and partly by the objects of the key. If the data in an
attribute is sufficiently detailed, it can uniquely identify a member. If not, you must add more
attributes ...
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