«ونوع: يكون الوقت معياراً للفعل، كالصوم»
Another type makes time itself the measure of the act, as in fasting.
«حكمه: إذا عين الشرع له وقتاً، لا يجب غيره في ذلك الوقت، ولا يجوز أداء غيره فيه، ويسقط شرط التعيين، كالصوم في رمضان»
Where the law assigns the time to a particular obligation, another fasting duty does not occupy that period in its place, and in the ordinary case the requirement of specifying which fast is being intended falls away.
This addresses the usual obligatory Ramadan fast. Travellers, ill persons, and other exceptional statuses have further jurisprudential detail.