What is meant by, "It is permissible to choose between parts of the time in an expanded obligation such as prayer, with no difference, due to the attainment of the benefit and the fulfillment of the duty," and what is meant by the attainment of the benefit and the fulfillment of the duty?

Obligatory duties, in terms of their timing, are divided into two categories: constricted (Muḍayyaq) and expansive (Muwassaʿ). The constricted duty is one whose time is exactly coextensive with its performance, whereas the expansive duty is one whose time is wider than its performance, such as the times for prayers. The majority of jurists from the Mālikī, Shāfiʿī, and Ḥanbalī schools affirm the expansive duty, arguing that it is permissible intellectually and established legally, and by analogy with the discretionary duty (al-wājib al-mukhayyar) like the expiation for an oath. The expansive duty resembles the discretionary duty in that the مكلف (legally responsible person) has the choice to perform the act of worship in any part of the designated time, which fulfills the obligation and achieves the intended benefit.