The recent Dictionnaire des régionalismes du français médiéval de l’Est (DRFM) constitutes an analysis of 389 lexemes contained in the corpus Documents linguistiques galloromans (DocLing, including about 2,350 documentary texts). These words represent concepts linked mainly to administration, commerce, and agriculture in the Eastern part of the Galloromance area (Champagne, Lorraine, Bourgogne, and Franche-Comté). More than half of these words are also attested in over 500 non-documentary texts of religious, literary, and practical nature. Such texts are notoriously difficult to localize, however, the regional words they contain aid this process considerably. In the present article, we distinguish three types of lexical regionalisms: « formal », « semantic », and « integral ». In the first section, we clarify the theoretical and phenomenological aspects of this distinction. We discuss the various roles of lexical regionalisms in the localization of non-documentary texts and, more specifically, the relative usefulness of formal regionalisms in this process. In the following sections, we demonstrate two methodological uses of regionalisms in non-documentary texts: 1. the localization of medieval texts or manuscripts (we provide 24 examples) and 2. the clarification or the resolution of ecdotical problems during the preparation of critical editions.