Board Thread:Off Topic/@comment-24993958-20160711184711/@comment-26397825-20160923214849

MiniDaggers wrote: It seems more restricted, the Schnees (due to lack of info leading to assumptions) can presumably summon anything they defeat (kill?). This semblance is restricted to animals and thus can't create Grimm constructs. I believe the Schnees are actually restricted; they can only summon Grimm that have challenged them and pushed them to grow stronger.

From the transcript of Lessons Learned:

Winter : Excellent form! Now think to your fallen foes! The ones who forced you to push past where you were, and become who you are now. ( as the glyph flickers in and out of existence and Weiss grimaces in concentration ) Think of them, and watch as they come to your side.

So an enemy that doesn't really mean anything to them beyond being yet another enemy would be unable to be summoned, but an enemy who has pushed them would be. Such as the Giant Knight that Weiss fought in the White trailer, which according to the manga was an obstacle standing in her way of going to Beacon. If she didn't defeat it, she would have been stuck at Atlas and under her father's thumb for the year, which she refused to accept. So she pushed herself further and defeated it, and now she's working on summoning it fully.