  This thesis deals with General Probabilistic Theories (GPTs) and Entanglement
Structures (ESs). An ES is a possible structure of a quantum composite system
in GPTs, which is not uniquely determined as the Standard Entanglement
Structure (SES). It is an important problem to find reasonable postulates that
determine various ESs as the SES. In order to solve this problem, this thesis
explores the diversity of ESs. The topics of this thesis are roughly divided
into two parts. First, this thesis considers state discrimination in ESs. As a
main result, this thesis gives equivalent conditions for a given measurement in
ESs to have a performance superior to standard quantum theory. Second, this
thesis focuses on symmetry and self-duality. As a main result, this thesis
gives derivations of the SES by symmetric conditions. On the other hand, this
thesis clarifies that there are infinitely many self-dual ESs, even if they
cannot be distinguished from the SES by a certain physical experiment with
small errors.
