Although the importance of formative assessment has been recognized worldwide, the theoretical foundation is insufficiently captured within a broader sociocultural context that promotes teachers and students building an assessment culture. This study proposes a theoretical framework that supports the claim that formative assessment aims to accelerate an agentic process of transforming and improving the teaching-learning activity systems rather than helping teachers mold students with traditional values and cultural discourses. The characteristics of formative assessment were organized for each of the learning metaphors: acquisition, participation, and expansion. In this paper, assessment for expansion is defined as a form of formative assessment to facilitate expansive learning toward a process of making teaching-learning better, of which the functional core is sociocultural feedback with reference to situational criteria. Next, the theoretical discussions demonstrate that assessment for expansion emerges from making a third space and forming a culturally fitted tool for realistic and sustainable practical judgements. These conditions, which work within a continuum of problematic, ends-in-view, and expanded contexts, recognize the impact of assessments in associating a single student's voice with a school- and community-wide problem. In conclusion, the possibilities and challenges of assessment for expansion are discussed from theoretical and practical perspectives.