Station Limits set how many stations can run at once, at various levels.
Stack or Overlap means that each program can be manually set to overlap with other programs, or be required to stack. Programs that are set to stack can only run by themselves.
SmartStack specifies a maximum number of programs that are allowed to overlap across the whole controller.
Maximum Simultaneous Stations is the total number of simultaneous stations that can occur for any reason in the entire controller. This is mainly for use with the Flow Manager, but applies to all situations.
If Flow Manager is enabled, the display will also show Program Limits. This sets the max number of stations that can run within any one program. This might be used to force irrigation to be spread over a larger number of programs, when Flow Manager is scheduling stations on to reach a flow rate target.
ACC2 Decoder can operate up to 30 simultaneous stations per controller, but only 20 per decoder output module.