eXplain is a specialized code-analysis and legacy-system evaluation tool from PKS Software GmbH, designed to deeply analyze, map, document, and assess legacy applications, especially on mainframe platforms such as IBM i (AS/400) and IBM Z, so organizations can understand what lives in their software, how it’s structured, and what parts are worth keeping, refactoring or retiring. It imports existing source code into an independent “eXplain server”, no need to install anything on the host system, then uses advanced parsers to examine languages like COBOL, PL/I, Assembler, Natural, RPG, JCL, and others, along with data about databases (Db2, Adabas, IMS), job-schedulers, transaction monitors, and more. eXplain builds a central repository that becomes a knowledge hub; from there, it generates cross-language dependency graphs, data-flow maps, interface analyses, clusterings of related modules, and detailed object-and-resource usage reports.