Does a `tree:shape` define a shape that members MAY or MUST adhere to?

[rorlic]

The Overview section says:

tree:shape indicates the [SHACL] shape (exactly one) to which each member in the collection adheres

In the Vocabulary section:

• The tree:Collection class says:

A collection has members that MAY adhere to a certain shape

• The tree:shape property says:

The SHACL shape the members of the collection adhere to.

I interpreted the first and the last text as option 1.
I interpreted the middle text as option 2.
But, I can image it could mean option 3 as well, while I do not think option 4 is intended.

Options:

1. A collection (tree:Collection) MUST refer to one shape using its tree:shape property to which each member MUST adhere to
2. A collection (tree:Collection) MUST refer to one shape using its tree:shape property to which each member MAY adhere to
3. A collection (tree:Collection) MAY refer to one shape using its tree:shape property to which each member MUST adhere to
4. A collection (tree:Collection) MAY refer to one shape using its tree:shape property to which each member MAY adhere to

Which leads to the question: which option is it? 🤔
Can we clarify this in the specification?

BTW. if a collection MAY refer to one shape, maybe it is better to recommend is using a SHOULD. What say you?