These are two types of Event Manager queues, Asynchronous (async) and Synchronous (sync). Async queues are executed as soon as possible with no guaranteed order. Sync queues are executed serially. If you have multiple tasks set to run on one sync queue they will execute one after the other in the order that they were put in to thee sync queue. The EM treats sync and async queues differently.
In Sync Queue each task in a sync queue must be executed in serial. To prevent problems in a cluster, an EM will claim ownership of one or more sync queues when it starts up. The ownership is stored in the LSW_UCA_SYNC_QUEUE where QUEUE_OWNER is linked to OWNER_ID in LSW_EM_INSTANCE. This is not a permanent assignment. The LSW_EM_INSTANCE table keeps track of status of all of the event managers. The status is checked every 15 seconds by default. If the owner of a sync queue is no longer available another EM will take ownership of that sync queue.
In Async Queue tasks are picked up by each EM when there is room in their async queue for more tasks. Each process server has it’s own running EM. The EM is configured by each process server’s copy of 80EventManager.xml: There are various EM settings in that xml file and they are all specified in milliseconds.