+replaced with the environment or macro.
+@end defopt
+
+@defopt reftex-trust-label-prefix
+Non-@code{nil} means, trust the label prefix when determining label type.
+It is customary to use special label prefixes to distinguish different label
+types. The label prefixes have no syntactic meaning in LaTeX (unless
+special packages like fancyref) are being used. RefTeX can and by
+default does parse around each label to detect the correct label type,
+but this process can be slow when a document contains thousands of
+labels. If you use label prefixes consistently, you may speed up
+document parsing by setting this variable to a non-nil value. RefTeX
+will then compare the label prefix with the prefixes found in
+`reftex-label-alist' and derive the correct label type in this way.
+Possible values for this option are:
+
+@example
+t @r{This means to trust any label prefixes found.}
+regexp @r{If a regexp, only prefixes matched by the regexp are trusted.}
+list @r{List of accepted prefixes, as strings. The colon is part of}
+ @r{the prefix, e.g. ("fn:" "eqn:" "item:").}
+nil @r{Never trust a label prefix.}
+@end example
+The only disadvantage of using this feature is that the label context
+displayed in the label selection buffer along with each label is
+simply some text after the label definition. This is no problem if you
+place labels keeping this in mind (e.g. @i{before} the equation, @i{at
+the beginning} of a fig/tab caption ...). Anyway, it is probably best
+to use the regexp or the list value types to fine-tune this feature.
+For example, if your document contains thousands of footnotes with
+labels fn:xxx, you may want to set this variable to the value "^fn:$" or
+("fn:"). Then RefTeX will still do extensive parsing for any
+non-footnote labels.