It's a question of awareness:
If she commits an act that she is not aware of which undermines your preference, then you overtly state your boundary, so she now becomes aware of your preference.
For example: I prefer that guests take off their shoes before entering my house.
Here, as she been to many houses and likely has never removed her shoes before entering, it's reasonable to assume she is not aware of this particular preference. An Overt boundary here would be appropriate.
However:
If she is aware of the "rules of play" in a normal healthy relationship, then overtly stating your boundary is implictly asking her to commit actions that contradict her desire. You can't negotiate desire. Hence, an overt boundary here is not your best play.
Instead, in such a context, you employ Silence and Distance (S&D), a covert boundary. You do this for two reasons:
1) She has made clear her desire contradicts yours. Hence, your desires are incompatible, and so you must walk away;
2) If her desire were to test you as your recent rapport with her has created doubt, S&D may forge a new desire in her, one that is compatible with yours. You employ S&D to walk away from an incompatible relation, not to change her, but her change is often a natural consequence thereof.
Here:
This is an act she is aware is wrong, and undermining. So S&D is appropriate.