وَالثَّانِيَةُ بِالتَّضَمُّنِ لِكَوْنِ الْجُزْءِ فِي ضِمْنِ الْمَعْنَى الْمَوْضُوعِ لَهُ
The second is called inclusion because the component lies within the meaning for which the word was established.
If the full essential meaning of “insān” is analyzed as “ḥayawān nāṭiq,” indicating only ḥayawān or only nāṭiq amounts to taḍammun. The complete meaning remains the word's assigned sense, but the mind considers an internal constituent.
Likewise, “having three sides” is already inside the definition of “triangle.” Isolating that element is not the same as inferring some separate property external to the concept.