What is required for the intention of a specified voluntary prayer (Nafl)?

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Voluntary prayers are divided into specified ones, such as the prayer for an eclipse (Kusuf), prayer for rain (Istisqa), Tarawih, Witr, and regular Sunnah prayers, all of which require specification in the intention. Voluntary prayers that are unrestricted, such as night prayer (Salat al-Layl), are valid with the mere intention of 'prayer' without further specification because the act itself is not specific.