You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: A Survey of Delegation from an RBAC Perspective 
Chapter Title:  4) User-Role Assignment: 
1. Abstract of Cited Paper (BIB001):  Delegation in computer systems plays an important role in relieving security officer’s management efforts, especially in a large-scale, highly decentralized environment. By distributing management authorities to a number of delegatees, scalable and manageable security management functionality can be achieved. Recently, a number of researches are proposed to incorporate delegation concept into Role-Based Access Control(RBAC) model, which is becoming a promising model for enterprise environment with various organization structures. In this paper, we propose a new role-based delegation model using sub-role hierarchies supporting restricted inheritance functionality, in which security administrator can easily control permission inheritance behavior using sub-roles. Also, we describe how role-based user-to-user, role-to-role delegations are accomplished in the proposed model and analyze our delegation model against various delegation characteristics.

Literature Review:
User-role assignment can be used to decide if a user should be a delegatee. A basic assumption made by Barka and Sandhu  is that one user u 1 should not be allowed to delegate a role to another user u 2 if u 2 is already a member of the role. This is a property based on the user-role assignment relation. Another way of saying this is to require that the original members and delegated members of a role be disjoint sets. Also observe here that if a user is assigned to a role r 1 , they are implicitly assigned to all roles junior to r 1 , so there is no point delegating such a user to any of these junior roles; this is another way of stating that a user should not be a delegatee if they are already a member of a role. It is feasible to delegate this user to a role senior to r 1 , if they are not a member of this senior role. In several papers, the term role-to-role delegation is used to describe delegation of one role, say SAccnting, to all members of another role, say PAccounting  , BIB002 , BIB001 . This concept is best categorized as a property based on userrole assignment. The delegation has nothing to do directly with role-role relationships, but rather with the membership list of the role PAccounting being used to define possible delegatees. A relationship based on R × R can be used to specify a candelegate relation, and the property can also be expressed as a constraint. However, we stress that the fundamental concept being used is the user-role assignment. Two-step and multi-step delegation, where the original delegatee further delegates to another user, either once (giving two-step) or many times, is also based on user-role assignment; in this case a user who is a member of a delegation role (having been delegated to it) can further delegate the delegation role. Grant-independent revocation can be defined so that any member of the delegating role can revoke a delegation  . This also depends on the original user-role assignment. Of course, grant-independent revocation could also be a permission given to a super-user or administrator, which is based on being assigned to the appropriate administrative roles.