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Multimodal intelligent systems combine evidence from text, vision, audio, sensors, and structured data, but their explanations often remain fragmented into separate saliency maps, feature scores, textual rationales, attention visualisations, or confidence indicators. Such outputs may describe isolated aspects of a model while obscuring how modality-specific evidence was represented, transformed, fused, and communicated. This article develops a conceptual multimodal explainability architecture organised around the principle of following evidence across modalities. The approach begins by distinguishing the evidential and representational characteristics of textual, visual, acoustic, temporal-sensor, and structured inputs. It then identifies attribution difficulties created by early, intermediate, late, and hybrid fusion, including modality dominance, missing evidence, conflicting inputs, representation alignment, and uncertainty about joint contributions. Evidence traceability is treated as an end-to-end requirement connecting input provenance, modality-specific representation, fusion operations, attribution processes, consistency assessment, uncertainty communication, and decision explanation. The architecture further incorporates human interpretation because technically detailed multimodal explanations may remain cognitively inaccessible, visually persuasive, incomplete, or poorly calibrated to user expertise and decision context. Its central contribution is a layered framework linking modality inventory, evidence classification, representation mapping, fusion attribution, provenance, consistency review, human-centred presentation, evaluation, and feedback updating. The architecture is intended to structure research and design reasoning rather than prescribe a particular model, explanation technique, evaluation metric, or deployment procedure. It does not establish that an explanation is faithful, that users understand it, that reliance is appropriate, or that the underlying intelligent system is trustworthy or ready for operational use.

Trustworthy artificial intelligence is frequently discussed through principles, performance measures, governance commitments, or general descriptors such as safe, fair, explainable, and responsible. These expressions are important but insufficient for determining whether a particular intelligent system is adequately justified for a defined use, environment, population, and decision context. This article proposes a conceptual AI assurance-case methodology for organizing trustworthiness reasoning through explicit assurance claims, decomposed subclaims, bounded acceptance criteria, evidence requirements, evidence-quality assessment, provenance, traceability, structured argumentation, defeater analysis, uncertainty, and residual-risk reasoning. The methodology responds to distinctive assurance challenges created by model opacity, distributional change, context dependency, conflicting trustworthiness objectives, incomplete evidence, human-oversight limitations, and lifecycle evolution. It distinguishes the existence of evidence from its relevance, validity, credibility, completeness, and sufficiency, while treating traceability as an aid to inspection rather than a guarantee of evidential quality. Structured arguments connect claims to evidence through warrants, assumptions, contextual conditions, and inference links, while counterevidence, argument gaps, evidence gaps, and unknown risks remain visible. Assurance cases may consequently support accountable deployment deliberation by clarifying decision gates, deployment boundaries, monitoring obligations, and conditions requiring reassessment. The principal contribution is an integrated methodology for constructing and maintaining bounded trustworthiness cases without treating assurance reasoning as proof of safety, fairness, legality, ethical sufficiency, compliance, certification, operational readiness, or deployment acceptability. The methodology supports transparent and challengeable reasoning, but requires domain-specific evaluation, governance review, stakeholder participation, and continuing lifecycle monitoring.