One of my PCs has the ability to see into the Ethereal Plane, but I'm not exactly sure what that entails.

Obviously, he can see ethereal creatures (Phantasmal Warrior, etc.), but is that the extent of it?

One of my PCs has the ability to see into the Ethereal Plane (using the spell True Seeing), but I'm not exactly sure what that entails.

To clarify, the spell has multiple effects, most of which are self explanatory:

For the duration, the creature has truesight, notices secret doors hidden by magic, and can see into the Ethereal Plane, all out to a range of 120 feet.

But I'm specifically wondering about what it means to be able to "see into the Ethereal Plane".

Obviously, he can see ethereal creatures (Phantasmal Warrior, etc.), but is that the extent of it?