The superlative takes different forms when used as a predicative or when used as an attribute. When used as a predicative, it is preceded by «am»:
- Sie ist am schnellsten.
- Er schwimmt am schnellsten.
When used as an attribute, there is no «am»:
- die schnellste Frau
- der schnellste Schwimmer
With regard to the declension, a ‘substantivized’ adjective behaves exactly like an attributive adjective, except it has no following noun:
- die Schnellste
- der Schnellste
This means that grammatically speaking, the ‘substantivized’ adjectives are normal adjectives. Nouns decline according to case and number; adjectives decline not only according to case and number, but also according to gender and definiteness – and so do ‘substantivized’ adjectives.
In the definite nominative singular, for instance, the ‘substantivized’ adjectives have different forms for all three genders:
- Liebster – Liebste – Liebstes
These are exactly the same forms as for ‘non-substantivized’ adjectives:
- liebster Mann – liebste Frau – liebstes Kind
Another example: In the masculine singular nominative, ‘substantivized’ adjectives alternate between definite (or strong) and indefinite (weak) endings:
- Liebster – der Liebste
Again, these are exactly the same forms as for ‘non-substantivized’ adjectives:
- liebster Mann – der liebste Mann
And a third example: In the masculine singular dative, ‘substantivized’ adjectives alternate between definite (or strong) and indefinite (weak) endings:
- auf Grossem – auf dem Grossen
Yet again, these are exactly the same forms as for ‘non-substantivized’ adjectives:
- auf grossem Fuss – auf dem grossen Fuss
So grammatically speaking, ‘substantivized’ adjectives are just regular adjectives with all the quirks of German adjective declension (sorry). The only difference between a ‘substantivized’ adjective and a regular adjective is the capitalization.
In my opinion, the definition of ‘substantivized’ adjectives is quite circular: An adjective is capitalized when it is ‘substantivized’, and an adjective is ‘substantivized’ when it is capitalized. Sure, you could try and formulate conditions when an adjective without a following noun counts as ‘substantivized’, but you would soon get lost in all kinds of special cases and exceptions. And even the relevant paragraph 57 of the German orthography rules says that ‘substantivized’ words have noun features (not grammatically though, as I have shown), and it links to paragraph 55 which states that the feature of nouns is their capitalization.