This param should allow to filter out chats which will be handled by the ChatJoinRequestHandler, much like the pattern argument of the CallbackQueryHandler. It should allow "username" strings as well as ids and if set, the handler should check if the incoming update is from that chat.

For first time contributors, check how CallbackQueryHandler implements the pattern argument in check_update: https://github.com/python-telegram-bot/python-telegram-bot/blob/master/telegram/ext/_callbackqueryhandler.py#L123