In the daily work of a software architect, knowledge is ubiquitous. In addition to technical expertise, architecture knowledge (AK) also requires practical experience in the representation, communication, and management of architectural decisions. However, there is a pressing need to also incorporate sustainability aspects, i.e., capturing decisions towards software systems that are environmentally, economically, and socially balanced in the long term. With this study, we aim to provide a review of AK concepts and their representation and communication from a practical point of view. Having this understanding, we explore where sustainability can be applied in daily practice and how we can address sustainability in architecture processes in the future. The paper presents an empirical study conducted in an industrial context encompassing a questionnaire survey with 32 participants and semi-structured interviews with 15 practitioners; both groups are from a major bank in the Netherlands. Based on the insights gained from combining our findings, we (i) provide a map of applied concepts for communicating and representing AK in a large enterprise, and (ii) discuss potential avenues for carving sustainability into current software architecture practice.