Thrombus formation in the context of STEMI is fibrin-rich; it causes coronary artery occlusion leading to myocardial ischaemia and subsequent infarction. This manifests electrocardiographically as ST segment elevation with a distribution of changes depending upon the coronary artery affected.
A thrombus occurring in the context of NSTE-ACS is platelet-rich; spontaneous thrombolysis and fragmentation into smaller particles release platelet emboli, which may cause small areas of more distal infarction (micro-infarction) without complete occlusion of the coronary artery. This is the process that is thought to be occurring in infarction without ST elevation, i.e. NSTEMI.