parse (syntactic analysis) - as result of this phase the AST is created and syntax errors are reported. You can think of this phase as a step enabling the usage of the com.sun.source.tree package. However, please note, that at this point the AST is NOT resolved, i.e. no type information is available. This mean that you can’t tell at this point of what type an identifier is. For example you will be able to find out that there is a field of some name in the class, but will not be able to find out what class(es) can be assigned to the field. This may seem to be an uninteresting phase but, for example, some code completion code does not require type information, so keeping the javac in the parse phase speeds up the task’s completion a bit.

elements resolved - the type information is added for classes (not for local classes - innerclasses contained in bodies of methods) and their members, elements (see below) are attached to the tree. It is possible to get (resolved) return type of a method, get a type of a parameter, field, etc. The content of methods is not resolved at this phase, so no type information about the statements is available. This is the correct phase for those who want to work at the level of class, methods, fields, and other signatures but who are not interested in the code in method bodies. Yes, there are such modules. Think about stuff like JavaBeans support or a module working with Javadoc.

resolved (aka attributed) - the type information is resolved for the whole source code. You can get type information for expressions inside the methods, etc. If you need to get all information about the code then this is the right phase for you.