https://help.maxon.net/zbr/en-us/Content/html/user-guide/zbrush-other-programs/zapplink/zapplink.html

While in ZBrush, call up your image editing package and use it to modify the active ZBrush document or tool, then go straight back into ZBrush.

ZAppLink can work on different saved points of view for your model. What you paint in your image editor is then projected to the model’s PolyPaint or texture for more creative freedom.

With ZAppLink you can combine ZBrush’s powerful capabilities with all the painting power of the PSD-capable 2D editor of your choice, making it easy to create stunning textures.

ZAppLink features

• Send your document view to the PSD file editor of your choice for texture creation and modification: Photoshop, Gimp and more!
• Projections in orthogonal or perspective mode.
• Multiple view support: With a single click, send your front, back, left, right, top, bottom and two custom views in dedicated layers to your 2D editor. When your painting is done, automatically reproject all the views back in ZBrush!
• Create character sheets based on your saved views with a single click.
• ZAppLink works with PolyPaint, Textures based on UV’s and canvas pixols.