The markers 'i' and 'ki' cover a range of meanings and satisfy a range of conditions. Most commonly the objects of 'i' are obligatory, while only some objects of 'ki' are.
The symbols 'i' and 'ki' signify different types of things that can be used in a certain context.  Most often, the things targeted by 'i' are required, while only some of the targets for 'ki' are.