Tombow ABT pens have two tips - one fine tip, and one soft brush-shaped tip. Use the fine tip for details, places where you need a predictable line width, and for stencils. The brush tip can be used for more painted effects, and for hugely varied and expressive lines.
Water-based and water-soluble ink means these pens can be blended with plain water - no need for special blending 'ink'. The blending pen is a convenient way to do this anywhere, but any brush can work with some water for blending. Make custom mixes of colours by scribbling with different pens on a non-porous shiny surface, and using any brush to mix the colours for the result you want. Tombow's own blending kit is the most convenient way to do this, and includes a blending brush, water spray bottle, palette and guide.