See the other Streams, Advanced Concepts pages.
Groups can be used to allow/refuse more than one source of data per group. If two wireless devices both have their Group et to “1” then even if two eligible heart rate straps are in range, the first to connect means that the other can’t. This is useful in situations where only one person at a time should be monitored, or multiple people but not two of the same type at a time. E.g. driver and co-driver should be monitored at the same time but only one of each is ever permitted. For a yacht, it may be that tens can be monitored but you want to avoid each crew’s backup device if the primary is already in use.
Leaving a group at zero means this setting is ignored but setting it to non-zero means that only one configured item per group can connect. i.e. if you have five devices all in group zero then if all the devices are present. If they are in a non-zero group then only one per group can be connected to at any one time.
Non-zero group numbers have no numerical meaning beyond associating devices together. i.e. Group 1 behaves as, say, Group 55. There is no interrelation or priority between groups, they are separate entities.