Sant Bhasha is not a language with a territory. It is the composite devotional idiom used by the North Indian sant poets, drawing on Braj, Khari Boli, Punjabi, Persian, Arabic and Sanskrit without settling into any one of them, and it exists because the poets who used it were addressing people across a wide region and several communities at once. Scholars sometimes call it the old western Hindi of the sants, and no two compositions in it draw on quite the same mixture.
Its most important body of text is the Guru Granth Sahib, the Sikh scripture, whose compositions are largely in this idiom rather than in Punjabi as it is ordinarily spoken, although it is written in the Gurmukhi script. Pashaura Singh's study of the canon traces how Guru Arjan compiled it in 1604 and how the final recension came to be fixed, and the volume includes not only the Sikh Gurus but Kabir, Namdev, Ravidas, Farid and others, so that a Sikh scripture carries the words of Muslim and Hindu figures inside it as scripture rather than as quotation.
That inclusiveness is a doctrinal position and not an editorial convenience. The idiom itself is part of the argument. A poetry composed in a language belonging to no single community suits a teaching that rejects the authority of birth, of caste and of a priestly tongue, and the sants used it in that way deliberately.