"upper and lower level limits"
The "upper" and "lower" are assembled into a group, which then radiates the properties of the members of the group.
"lower" is a verb and an adverb, as well as an adjective like "upper".
The objects in the group are evaluated as to part of speech, and only the parts of speech that are common to all the objects in the group are radiated, preventing inappropriate parts of speech from being radiated. This is of relevance only during parsing - if the information about part of speech is not required, this functionality is turned off.
Not all text follows this rule - an example of an indexed list:
The requirements are:
a) unable to remain consistent.
b) tending towards complexity.
c) incoherent.
d) mainly constraints.
The "are" before the colon is variously a verb auxiliary and a verb. The initial word in the indexed list is (ignoring adverbs) variously an adjective, a verbal participle, a noun.