The bollen (sometimes spelled as bolleen, boline or bolline) is more commonly called the white-handled knife nowadays. Traditionally it was used for herb harvesting and had a small curved blade.

Today the blade is rarely curved but is still has a single cutting edge. It has a white handle to differentiate it from the athame. Some witches will make do with a store bought white-handled knife while others will buy a purpose made tool.

The boleen is used for cutting and carving. Branches for wands for instance, should be cut with a boleen. The carving of symbols (into a candle for example) should likewise be done with a boleen. It is also used for cutting cords for future use in cord magick.

The boleen can be used for any ritual function that requires the physical use of a knife. Cutting wood for wands, flowers for the altar, carving and so on. As long as it's ritually involved it's fine. So unless potatoes are part of one of your rituals, don't use a boleen to peel them!