Mandatory events: you will be informed regarding this type of corporate action, however this will only be for information purposes as you will not have a choice whether to participate in the corporate event or not. Once this type of event has gone into effect in the market, it is applicable to all outstanding shares or bonds, the investors will not have a say in it. These types of corporate actions will be applied and booked automatically and do not require a response from you. Examples of mandatory events are spin-offs and stock splits.
Corporate actions with choice options: these are mandatory events which require a choice by the investor. An example of this is the choice on how to receive dividends. The dividend will be paid regardless (even if you do not respond) however you have the choice between receiving the dividend in cash or in shares. If you do not send instructions, you will automatically receive the default option which will always be described in the corporate action notification.
Voluntary events: for voluntary events you have the choice whether to participate or not. The investors together actually have a say in the outcome of the voluntary corporate action. An example of a voluntary event is a public takeover bid, every investor has the choice to offer their shares to the company that launched the bid or to not offer their shares. In this case if all investors refuse to offer their shares, the takeover will not take place.