Fraction absorbed (Fa) is an important parameter to describe the absorption level of oral drugs, and an important basis for the development and optimization of the formulation process. Because it is easily confused with the concept of absolute bioavailability, it has not received enough attention from the industry. There are many complex factors affecting Fa. There are three time-related factors that directly affect the extent of Fa: the release time, the absorption time, and the residence time. The relationship between these three time-related factors determines the extent of Fa. Generally, we are more concerned about the apparent factors that affect the extent of Fa, including independent variables and covariates; The independent variables include administered dose, route, dosage form, etc. The covariates are divided into internal and external factors, and external factors include food factors, drug interactions, etc. Internal causes include age, sex, disease, etc. This paper analyzes and systematically combs how independent variables and covariates directly or indirectly affect the three time-related factors by affecting the body's physiology and internal environment, thus changing the complex process of Fa. Understanding this theoretical framework can better optimize the independent variables to reduce the impact of covariates on Fa. In addition, this paper also introduces the latest progress of prediction and evaluation of Fa, including the progress of complex dissolution device and the status of software prediction.