The top part of a C-shaped arc, after skis flat to the fall line. Sometimes referred to as the "initiation phase", including some pivoting and steering, but when referring to the top of the turn as the "High C", the focus is on edging the skis, moving the body inside the turn, flexing and extending and progressively increasing weight on the new outside ski, to engage the edge.
The way you use the top of the turn is important for the level of engagement and carving that follows. It is critical on ice and such, learn more in The High-C. One of the critical movements in the high-c is extending++ the new outside leg, to maintain edge-snow contact, build edge angles and start engaging the edges, frames 3-6 here:
However, during this extension it is important to try to tip the skis to as high an edge angle as possible before pressuring, because when you push into the ski it tends to stop tipping and start to flatten:
The steeper the hill and the sharper the turn, the more edge angle is required to prevent skidding when you first pressure your ski.The Athletic Skier, Warren Witherell, 1994
Often referred to as "creating the platform", the main goal at the top of the turn is establishing a "solid platform" by engaging the edges of the skis to start carving. Of course, being forward and recentering is important when engaging the edges to start carving, so the engagement starts with the tip of the skis, for performance carving and a powerful engagement.
The Pressure increases during this engagement, but it is still very light compared to the apex and the bottom of the turn, so a good feel for the edges is important, to establish a good solid engagement rather than jerk the edges out of engagement and skid the top of the turn.
You can hear Ted Ligety comment about skidding vs carving the top of the turn, where the "sliding makes it much easier to not make mistakes, but it's a definitely a lot slower from turn to turn":
Certainly however, all those are valid tactical choices, given the skill and conditions.
Reading and progressions:
Establishing a good edge angle early in the turn, with early edge engagement is a critical skill for expert skiing.