Quality-of-life (QoL) assessment tools, such as patient-reported outcome measures (PROMs) are increasingly used in palliative care (PC) to reflect patients' voices. Contemporary measurement validity theories suggest that validation practice involves accumulating and synthesizing evidence to support interpretations and actions based on measurement scores. Measurement validity theories have been developed more often for psychometric purposes, including aggregate-level interpretations of scores, in education and psychology. Therefore, a knowledge gap exists for the clinical applications of QoL assessment tools, including in PC. The study aimed to understand how patients and PC professionals enacted validation practice in the clinical applications of QoL assessment tools in PC. Using an interpretive description approach, we observed participants using QoL assessment tools during routine care, and we individually interviewed eight patients living at home with life-threatening illnesses and six PC professionals. The data were analyzed diffractively to tell parts of a story about validation practice in the clinical applications of QoL assessment tools in PC. Validation practice is not a homogeneous phenomenon. Validation practices can be enacted by the relational use of QoL assessment tools; by focusing on an ongoing reflective dialectical interpretation of the scores with patients; by ascertaining with patients which priorities are most important to them; by skillfully using relevant metaphors to recognize the patient as a person; and by asking further questions to determine the patient's goals of care. As a boundary object with agency, a QoL assessment tool collaborates with people to create practice-based validity evidence. Diffractive analysis of socio-material practices enabled the observation of interference patterns and the consequences of using these tools. Validation practices are important for a person-centred approach to PC. Without a person-centred approach to using QoL assessment tools in PC, professionals risk intervening (or not intervening) based on assumptions rather than justifications.