The code is created by Emily and Riley at some point after they finally get back on good terms with each other. It is introduced when Emily receives the message while cleaning up after J-Troupe auditions in Studio A. Emily uses the code to contact Riley when her sister is at Mr. Abernathy's CPR class with James after being suspended from The Next Step Dance Studio by Kate on suspicion of stealing the Nationals money.
James is the only character other than the sisters to use the message. Riley receives the message from him while in Hidalgo's with Amanda. Riley discovers that he sent the message in his desperation to find a suit at Enzo's warehouse. Hence, James confesses that the message stood for "Seriously Need Suit", and definitely not "Sister Needs Sister".