Cases allow you to subdivide your work with a single contact on a single project into smaller components.
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For example, if you are provide legal aid to inmates in different prisons, each prison you work in is a project and each inmate is a contact. If the inmate is in prison on account of multiple charges, you can create a case for each charge.
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You can
decide the fields that are used to create each case
assign case workers (users) to a case
track the amount of time spent on each case
see all the case notes related to each case
report on the number of cases closed, opened and worked on, as well as several other metrics
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