To quote PE from another thread...

Quote Originally Posted by Photo Engineer
There are at least two types of direct reversal emulsions. The process is Develop, Stop or Rinse, and Fix using normal chemistry.

The two types in common use are Reversal F and Reversal P which differ considerably. These emulsions are VERY complex and beyond the scope of any discussion so far on APUG, but to make it very simple, A reversal F emulsion is usually say a pure chloride cube which is then treated with bromide and made into an octahedron kind of shape. The cur of this, the chloride, may be fogged, sensitized some way or just left as is before shelling. This process is called conversion and does not make a true core shell emulsion.

That final emulsion then is sensitized as normal but uses a special chemical called a nucleating agent added to the mix.

At exposure, exposed areas will not develop, but unexposed areas will develop thus giving us a positive image directly with a normal developer.

There are two disadvantages. One is that the nucleating agent can build up in the developer changing both direct reversal results with time, and also it can affect negative emulsions so the developer is best used one-shot or only for direct reversal emulsions with lower capacity. The second disadvantage is that these materials have a shorter tone scale and higher fog than normal materials. This results in re-reversal or combined negative and positive images in the same frame. This is rare, but very ungood when it takes place.

Reversal F was used in Kodak PR-10 instant films and in Ektaflex R material.

This is from this thread.