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By any relevant skills/qualifications they might have
By the position they are assigned in their own profile
By the event-type & associated default event team/position
These can be setup in the “Event Teams/Positions” page, in the “Defaults” tab at the top-right of the page.
By skills/qualifications
Certain skills/qualifications can be taken into account - eg. if a person has a “Driver” skill/qualification, and there is a “Driver” event/team position at the event, the system will select that team/position by default.
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A “Local event” event-type might have a “Support” person which is selected by default when people are rostered
A “Corporate function” event-type might have a “Catering staff” position which is selected by default
etc
To setup this behaviour, the default team/position you would like to use needs to be selected against each event-type you have setup.