Library Content Size (C9 Pro) ?

refer to :

Does anyone know, what the (Cubase Pro 9) reference is on the above SB page?

What does this mean for users that download the [free] HS3-SE, that there is no content initially ?

It says that on the comparison page you referenced. When you install Cubase 9, there is some content such as GM sounds, currently playable with SE2, which presumably would be replaced by SE3 if installed, when that becomes available. So it looks like the downloadable SE3 will be a player only, with no sounds, although H6 can produce libraries for it, so I’m expecting there will be many free libraries very soon.

Yes, exactly. It is stated on the product comparison page:

The free HALion Sonic SE download version does not contain any content.

The free version will be the player software/plugin only. The usual Steinberg content is only available with a license of Cubase LE/AI up to Pro - and of course Nuendo+NEK, which come with Halion Sonic SE.