BI 0
Break indices of level 0 (BI 0) are used to mark cohesion between orthographic words. Orthographic words separated by BI 0 constitute a prosodic word, PrWord that may bear only one pitch accent.
BI 1
Break indices of level 1 (BI 1) mark boundaries between prosodic words, PrWords. Items separated by BI 1 should carry at most one pitch accent each.
So both BI 0 and BI 1 separate orthographic words, but words separated by BI 0 may bear only one pitch accent.
Example BI 0
Example BI 1
Que l'hi duries?
[Would you take it to him/her?]
Què li duries?
[What would you take to him/her?]
Explanations
These two examples correspond to very similar sentences but with two different prosodic organizations. The first one is a falling yes-no question introduced by the unstressed particle que, and the second one, a wh-question introduced by the wh-word què. Both contain three orthographic words.
In the first one, there is only one pitch accent, and hence, only one prosodic word (PrWord). The unaccented words have to be combined with the accented one to form a PrWord. So the boundaries between them are to be labelled with a BI 0.
In the second example, there are two pitch accents, so there are two PrWords. Què receives a pitch accent and forms an independent PrWord. On its right edge, there must be a BI 1.
On the contrary, like in the first example, the clitic pronoun li can't be accented and needs the verb duries to conform a PrWord, so the boundary between these two orthografic words has to be labelled with a BI 0.
