To define a dictionary format with the file extension .abc, add this name (without the .) as an entry point:

plover.dictionary =
  abc = plover_my_plugin.dictionary:ExampleDictionary

Dictionary plugins are implemented as classes inheriting from StenoDictionary. Override the _load and _save methods at least to provide functionality to read and write your desired dictionary format.

# plover_my_plugin/dictionary.py

from plover.steno_dictionary import StenoDictionary

class ExampleDictionary(StenoDictionary):

  def _load(self, filename):
    # If you are not maintaining your own state format, self.update is usually
    # called here to add strokes / definitions to the dictionary state.

  def _save(self, filename):

Some dictionary formats, such as Python dictionaries, may require implementing other parts of the class as well. See the documentation for StenoDictionary for more information.

Note that setting readonly to True on your dictionary class will make it so the user is not able to modify a dictionary of that type in the UI.