Dear Bumba,
“non-weighted” B&B is one of the four ways of estimating the prior based on the strength of B&B (the others being: histogram, weighted B&B and concept map). The “non-weighted” is the easier one to calculate because it consists in simply counting the number of barriers and the number of boosters, the assumption being that each B & B have the same strength (impact) on coverage (which is unlikely to be true in the reality). For example if you have 20 boosters and 15 barriers, the strength will be of 20 and 15 respectively.