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2 The Language: Rationale and Fundamentals 91
The YAWL Organizational model takes the form of a tree, based on the reporting
relationships between groups, where the most senior group within the organization
is the root node of the tree. This model is deliberately chosen to be simple and
generic so that it applies to a relatively broad range of situations in which YAWL
may be used. Finally, the Work distribution model is presented that captures the
various ways in which work items are distributed to users and any constraints that
need to be taken into account when doing so.

Definition 16. (Work distribution model) Within the context of a YAWL net nid,
it is possible to describe the manner in which work items are distributed to users for
execution. A work distribution model is a tuple (ResourceVarID, Auto, T M , Initia-
tor, DistUser, DistRole, DistVar, OrgDist, CapDist, SameUser, FourEyes, UserSel,
UserPriv, UserTaskPriv) as follows:

(* work allocation *)
ResourceVarID VarID is the set of variables that identify resources or roles;
Auto T A is the set of tasks that execute automatically without user intervention,
where T A is the set of atomic tasks;
T M T A nAuto is the set of atomic tasks that must be allocated to users for
execution;
InitiatorW T M ! fsystem; resourceg fsystem; resourceg fsystem;
resourceg indicates who initiates the offer, allocate, and commence actions;
DistUserW T M P.User/ identifies the users to whom a task should potentially
be distributed;
DistRoleW T M P.Role/ identifies the roles to whom a task should potentially
be distributed;
DistVarW T M P.ResourceVarID/ identifies a set of variables holding either
user or roles to whom a task should potentially be distributed;
dom(DistUser), dom(DistRole) and dom(DistVar) form a partition over T M I
OrgDistW T M OrgExpr identifies the organizational criterion that users who
execute the task must satisfy;
CapDistW T M CapExpr identifies the capability that users who execute the
task must possess;
SameUserW T M T M is an irreflexive function that identifies that a task should
be executed by one of the same users who undertook another specified task in the
same case;
FourEyesW T M T M is an irreflexive function that identifies a task that should
be executed by a different user to the one(s) who executed another specified task
in the same case;
UserSelW T M frandom, round-robin-time, round-robin-freq, round-robin
-exp, shortest-queueg indicates how a specific user who will execute a task should
be selected from a group of possible users;