how to assign specific user to next task

You have two different options to assign a task to a user:

• Actor mapping: http://documentation.bonitasoft.com/product-bos-sp/actors
• Filter: http://documentation.bonitasoft.com/product-bos-sp/actor-filtering

With actor mapping, your process definition will only embedded an actor id (a simple name) and after deployment the actor id will be mapped to your company organization stored in Bonita. The actor can be mapped to groups/roles or a list of one or several individuals. Main advantage of this solution is that you can update actor mapping configuration after deployment even for running instance. Limitation is that it cannot use any process instance dynamic information (e.g. id of the user who start the process or perform a task) nor external systems.

Filters (name is actually not appropriate) is a piece of code that will run when process execution reach the step. The piece of code will produce a list of Bonita users ids (can also output only one user id). Main advantage of this options is that it can use any process instance information and also external systems and can implement any kind of business logic. Limitation is that code will only run once when we reach the step. It requires some API calls to run again a filter.

So if in your use case performer of Step A need to choose the user who will perform Step B, you should go with a filter.