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While starting Extended advertisement, the host has to specify the primary advertising channel map. With regard to secondary channel map, there is no exact specification. It is rather implementation dependent. The controller may use all the secondary channels or use only those specified by the host using channel map update command.
For example, host can tell the controller to perform primary advertisement on channels 37 and 38 (not on 39) and perform secondary advertisement on channels x,y,z.......
These are the "primary and secondary channel indices" specified by the host. The channels to be used are not specified before every event.
In every advertisement event, the primary advertisement is done on all the primary channels specified by the host (amongst 37, 38 and 39th channel). The secondary advertisement channel can change from ADV event to event. However, the host need not command the secondary channel to be used. The controller determines the secondary advertisement channel to be used. This will take very less time and won't affect the advertisement timings anyhow.