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A common approach is to turn the cursor into a "can't drop" cursor when you're over everything else. The logic would be: Start Drag. Turn cursor into "can't drop". While dragging check if over something that can accept the drop then turn cursor into "can drop". Drop & reset cursor. NOTE: I'm not necessarily advocating this, but it does show the user ...
You can slightly highlight the drop-locations as soon as the user is dragging an item. This way you can draw the attention to the drop-locations. The prerequisite for this is of course that you show what's draggable in the first place. There this questions with answer for that.
Indicate possible drop targets Show a live preview of the results Show what's being dragged in real-time If you can't drop something somewhere, explain why to the users in a popup near the cursor: Keep your drop targets big! Drag and drop is already an egregious violation of Fritz's law; no need to make it worse.
I'm assuming a user of the system can pick one of three paths: Start from the people and for each person see which actions are available on which account. Start from the accounts, looking/setting who has what access to it. Start from actions and see on what accounts they are available and who can perform them - this seems a bit less likely, but I might be ...
Sometimes the drop target is within a set of linearly arranged objects. After the item is dropped, objects following the drop target will move to make space for the new object. In that case, the insertion point is typically indicated by a thin line between the two objects that will separate to make space. For example, in almost any GUI that uses a mouse, ...
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