The LORA project concerns the design of an experimental platform for validating and evaluating the methods of task scheduling and resource assignment. The heart of the platform is the modeling of interactions between three fundamental constructs which are the processes, the resources, and the time. To represent the processes in the time, we introduce a general and original model (the "task chart"), which allows for representation and exploitation of the flexibility of a process with processing alternatives. This model is the first one to integrate in the same graphical formalism, three types of requirements: an exact representation of temporal constraints between two events (starts/ends of tasks), a combined representation of alternative execution paths and of sequencing constraints, and representation of group relations of the composition-abstraction type (tasks, macro-tasks). This model of task chart also provides a generic and practical way for describing the resource-time and resource-process relations.