Why aren't all of the snap modes available for all tools?
The basic snaps are available for all tools. These include keypoint, nearest, intersection and so on.
The basic snaps determine the location of a point and can be computed by MicroStation without considering what tool is running.The constraint snaps are available only for the following tools: place smartline, line, arc, circle, ellipse.
Constraint snaps determine the geometry of a line, arc, circle, or ellipse.
They are part of the tool that is creating the geometry.
https://communities.bentley.com/products/microstation/w/microstation__wiki/16851/tentative-snap-not-working
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