جِهَةٌ جَامِعَةٌ: أَيْ مُنَاسَبَةٌ تَامَّةٌ
A unifying relation is a sufficiently close meaningful connection between two sentences.
It may arise from comparison as well as similarity. “Truthfulness benefits” and “Lying harms” contrast, yet share the theme of moral qualities and their outcomes. “Zayd studies” and “The river is deep,” without context, lack that obvious connection.
Suitability concerns sense, not similar wording alone. Contrasting propositions can be appropriately joined when their contrast is itself relevant.