«لا يكون الفرع منصوصًا عليه»
The subsidiary case must not already have its own express textual ruling.
A far‘ is a candidate for analogy because its ruling has not been determined for that case by a direct text. If both cases are explicitly addressed, the relevant text governing each case takes precedence over a constructed analogy.
The textbook compares emancipation requirements in expiations for an oath, zihar, and accidental killing. Its point is that one textual expiation should not automatically have its conditions rewritten by analogy to a separate textual expiation.