Merger mode is only accessible for shapes inside a Group of shapes. Because a shape only merges with the previous one, merger mode is locked for the first shape of a group.
Merger mode can be changed either directly from the Element Tree or inside the Parameters Editor, and then quickly switched with the up and down arrow keys:
In the Element Tree:

Mergers let you create complex shapes and the transition between them. Like the Blending Mode, the merging rate is defined by a percentage.

• 0%: Not merged
• 100% Totally merged (sometimes display only the previous one)