Assessments in tertiary science subjects typically assess content knowledge, and there is current need to both develop and assess different forms of knowledge and skills, such as communications and digital literacies. A digital explanation is a multimodal artefact created by students to explain science to a specified audience, which is an alternate form of assessment that has potential to develop and assess these other important forms of knowledge and skills. This research draws from perspectives in multimodality, educational semiotics and science education to gain a better understanding of digital explanation as a form of assessment in university science. Data sources include digital artefacts (n = 42), task descriptions and rubrics and pre-/post-interviews (n = 21) with students who created them as a task in a university science subject. Analysis involved identifying the range of media resources used across the data set, seeking patterns in how multiple resources were used and exploring students' perspectives on the task, including their design decisions. A more detailed look at artefacts from three different science learning contexts illustrates that students base their design decisions on the content knowledge being represented, their technical capabilities to generate them and how to engage the audience. Students enjoy this form of assessment and feel that the tasks allowed them to demonstrate different sorts of capabilities than are normally assessed in their subjects. Recommendations for instructors provide guidance for considering this sort of task in tertiary science contexts.