When you are using PowerDesigner, you will need to edit such things as descriptions and annotations for objects, or generated scripts like SQL in the PDM or Java in the OOM
By default, all files are edited using PowerDesigner's internal editor. You can define an external text editor to launch automatically for editing various kinds of objects. You can define as many text editors as you need, and the same file extension can have several text editors allocated, with the first listed acting as the default.