WIP limits may be placed on a board's users to ensure that a individual is not assigned too many work items at any given time.  User WIP limits are specified within the User Access section of the Board administration module.  

To set a user's WIP limit, specify the number of work items that a user can be assigned at any one time.  Specifying a value of 0(zero) sets the user WIP limit to unlimited.  Exceeding the limits for that board requires a reason to be specified.  This information is communicated to all subscribers so that the entire team is aware of the override event.

WIP limits can also be placed on the lanes/sublanes/and swimlanes within the board.  These limit the number of work items that can be placed within the lane, sub-lane, or swimlane. All WIP limits are enforced at the level that they are specified but also roll up to the parent lane's limit. This roll up is performed all the way to the top level.  For instance in the example below, the parent lane ("In Development") has a WIP limit of 3.  The child lanes ("Ready" and "In Process") each have a WIP Limit of 2.  When a work item is moved into "Ready", first the limit is checked on this lane.  If the limit is not exceeded on this lane, the parent's WIP Limit is checked.  If any of the WIP limits are exceeded, an override reason must be supplied.  In this example, if another work item is moved into "Ready", the parent lane's ("In Development") WIP Limit would be exceeded.