Minecraft wiki entry for BE, associated text: Block entity data
Tags common to all block entities[show]
 CustomName: Optional. The name of this beacon in JSON text component, which appears when attempting to open it, while it is locked.
 Lock: Optional. When not blank, prevents the container from being opened unless the opener is holding an item whose name matches this string.
 Levels: The number of levels available from the pyramid. Immediately changes to the correct value if modified using commands.
 Primary: The primary effect selected, see Potion effects for IDs. Set to -1 when no effect is selected. Cannot be set to an effect which beacons do not normally use, otherwise immediately changes back to -1. Although Regeneration cannot normally be chosen as the primary effect, setting this value to 10 works and even allows Regeneration II to be chosen as the secondary via the normal beacon GUI.
 Secondary: The secondary effect selected, see Potion effects for IDs. Set to -1 when no effect is selected. Cannot be set to an effect which beacons do not normally use, otherwise immediately changes back to -1. When set without a primary effect, does nothing. When set to the same as the primary, the effect is given at level 2 (the normally available behavior for 5 effects). When set to a different value than the primary (normally only Regeneration), gives the effect at level 1.