Leave It Alone

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Hi Guys. I don't really know how to differ a single from a promotional single when it is released digitally. Nevertheless, a publication on NME [1] refers to the song as a "new single" and the second to be released from Petals for Armor. The song has been released digitally (as "Simmer") in (I asume) most of the digital platforms. So I don't know if we have to deem the song as a single or a promotional one. Thanks in advance!