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). !