Capitalization varies, so Subhanu Wa Ta'Ala, along with Subhanahu Wa Ta'Ala, are both accepted by those "who believe" as conveyed through different acronym methods - i.e., SWT/swt, SWTA/swta, swtA, etc.
The phrase (often abbreviated to "swt" or to any of the above mentioned acronyms) appears after the name of Allah in Islamic texts such as the Qur'an and the Hadith. Saying this phrase is seen as an act of reverence and devotion towards Allah among Muslims.
All monotheistic faiths believe that there is one God. In Islam, a monotheistic faith, God (in Arabic: "Allah") is the creator of the universe. Sometimes Muslims use other expressions when the name of God (Allah) is written or pronounced. Some of which are: "'Azza wa jal": He is the Mighty and the Majestic; "Jalla Jalaluhu": He is the exalted Majestic. S.W.T. (or "swt") is then the Latinized standard abbreviation for the words "Subhanahu Wa Ta'Ala".