When task descriptions are precise they can be analysed to yield a variety of insights about interaction, such as the quantity of actions performed, the amount of information that must be perceived, and the cognitive workload involved. Task modelling notations and associated tools provide support for precise task description, but they generally provide a fixed set of constructs, which can limit their ability to model new and evolving application domains and technologies. This article describes challenges involved in using fixed notations for describing tasks. We use examples of recognized tasks analysis processes and their phases to show the need for customization of task notations, and through a series of illustrative examples, we demonstrate the benefits using our extensible task notation and tool (HAMSTERS-XL).