Nexus allows users to delegate management of their calendar, tasks etc. to other Nexus users. For example, you may want to allow your assistant to schedule your meetings or you may wish to assign tasks to your assistant. This is achieved by using Delegation.
Please note, the concept of delegation is different from Sharing, which allows others to access a particular folder or feature, but does not allow them to act on your behalf. However, within iCal the procedures for sharing and delegating are almost identical, and when you add a shared calendar to iCal it will be listed as a delegate. As a general rule of thumb we can think of shared calendars or tasks as being those that you can read but not write, whereas delegated calendars or tasks are those which you can write as well as read, i.e. you can add new calendar items or To Do items.
To add a delegate using iCal, or to act as a delegate for someone else, you will need to have iCal set up to connect to your Nexus account. If you have not already done so, please follow the configuration instructions for calendaring using iCal on Lion.Sections in this document: