There exists serious miscomprehension in the open literature about the electromagnetic interference shielding effectiveness (EMI SE) as a critical index to evaluate the shielding performance, which is misleading to the graduates and newcomers embarking on the field of electromagnetic shielding materials. EMI SE is defined as the sum of three terms including reflection loss, absorption loss, and multiple reflection loss in the classical Schelkunoff theory, while it is decomposed into two terms named reflection loss and absorption loss in practice, which is called Calculation theory here. In this paper, we elucidate the widely seen misconceptions connected with EMI SE via theoretical derivation and instance analysis. First, the terms in Calculation theory are often mistakenly regarded as the approximation of the terms with the same names in Schelkunoff theory when multiple reflection loss is negligible. Second, it is insufficient and unreasonable to determine the absorption-dominant shielding performance in the case that absorption loss is higher than reflection loss since reflection loss and absorption loss cannot represent the actual levels of reflected and absorbed power. Power coefficients are recommended to compare the contribution of reflection and absorption to shielding performance. Third, multiple reflection effect is included in the definitions of reflection loss and absorption loss in Calculation theory, and the effect of multiple reflections on shielding property is clarified as against the commonly wrong understandings. These clarifications offer correct comprehension about the shielding mechanism and assessment of reflection and absorption contribution to the total shielding.