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For SCL, I-joists, and CLT, the program calculates deflection due to shear and adds it to the bending deflection, using the method described in Shear Deflection. The key parameters affecting shear defection are the shear stiffness GA and a shape factor k.
For glulam and sawn lumber, the effects of shear deflection are incorporated into the value of the published modulus of elasticity E, which in turn affects the bending stiffness EI, which is considered an "apparent" stiffness rather than a "true" stiffness.
For SCL products, manufacturers publish both true E and apparent E values. SIzer provides a choice in the Design Settings allowing you to choose whether to approximate shear deflection with apparent E, or calculate it using true E.
Apparent E is not appropriate for complex span and loading conditions, and may be conservative for simple conditions, so the use of true E is recommended and should be used if the data is available from the manufacturer.
When searching through sections for a passing design, as opposed to a detailed design check, apparent E is always used for SCL, and for CLT and I-joists, shear deflection is approximated.