مُضَافَةً means the noun must be the first term of a possessive/construct phrase (iḍāfa).
جَاءَ أَبُوكَ.
Translation: Your father came. Here «أَبُو» is the first term and «كَ» is the second, so the special pattern can apply.
جَاءَ أَبٌ، رَأَيْتُ أَبًا، مَرَرْتُ بِأَبٍ.
Translation: A father came; I saw a father; I passed by a father.
Without iḍāfa, «أَبٌ» declines like an ordinary noun with ḍamma, fatḥa and kasra.
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Noun Case Classes: Type 4 — The Six Nouns
مُضَافَةً means the noun must be the first term of a possessive/construct phrase (iḍāfa).
جَاءَ أَبُوكَ.
Translation: Your father came. Here «أَبُو» is the first term and «كَ» is the second, so the special pattern can apply.
جَاءَ أَبٌ، رَأَيْتُ أَبًا، مَرَرْتُ بِأَبٍ.
Translation: A father came; I saw a father; I passed by a father.
Without iḍāfa, «أَبٌ» declines like an ordinary noun with ḍamma, fatḥa and kasra.