Explanation
PEC is an embroidery data format from the Brother ecosystem. In practice, many users encounter it as a data block inside a PES file. It can contain stitch information, color data, and preview-related information used by machines or software.
Relationship to PES
PES is the format most users handle directly. The PEC part is one reason PES files can show previews and machine-readable stitch data. When software converts PES files into another embroidery file format, not every piece of preview or metadata is always preserved.
Practical relevance
You usually do not need to edit PEC data manually. It is useful to know the term when troubleshooting older Brother designs, checking why a preview is missing, or understanding why two PES files from different software versions may behave differently.
For organizing a collection, the visible file format, design size, color count, and stitch count are usually more important than the internal PEC structure.