The criterion for distinguishing between the claimant and the defendant has faced conflicting opinions among the jurists of Imami jurisprudence regarding the presentation of evidence. Therefore, elucidating the criteria for distinguishing between the claimant and the defendant in Imami jurisprudence is essential for the implementation of justice, conflict resolution, and the reconciliation of the opinions of jurists. Using a descriptive-analytical method, the results indicate that among the various definitions of the claimant and the defendant provided by the jurists of Imami jurisprudence, the perspective that regards custom (urf) as a determinative factor in identifying the claimant and the defendant is more robust.This is because, due to the lack of a concrete legal definition for the terms 'claimant' and 'defendant,' custom becomes the key determinant for these two roles, and the differences in definitions arise from the clarification and limitation of their customary and linguistic meanings. Nevertheless, a precise elucidation and limitation of this customary criterion, along with an analysis of disputes from both its Subject Matter and purpose, appears essential. The subject matter of a dispute is what the parties refer to; thus, a change in the appearance of the wording and the subject matter of the dispute can lead to entirely different outcomes. When it comes to bearing the burden of proof and taking an oath, either party can be the claimant or the denier. If the purpose of the dispute is the central factor in decision-making, there will only be one claimant and the other will be a denier, and Actions will be taken based on the principle of evidence. Clarifying the relationship between the relevant areas in jurisprudence and the two mentioned theories, and choosing the preferable theory, is instrumental in accurately identifying the claimant and the denier.