A buried ball is not always embedded
Long grass can swallow a ball without making it embedded under the Rules. For Rule 16.3 relief, part of the ball must be below ground level in its own pitch-mark from the previous stroke. An awful lie alone is not enough.
When the free drop is available
Start by distinguishing soil from grass. A ball sitting deeply between blades may still be entirely above ground level. A ball pressed into the earth by somebody’s foot is not in its own pitch-mark from the previous stroke either.
The normal free-relief permission applies in the general area, with exceptions. It does not cover an embedded ball in sand that is not cut to fairway height or less. Nor is relief available when something else makes playing the ball clearly unreasonable—for example, its position inside a bush.
When relief is allowed, the reference point is immediately behind the embedded ball. Drop the original ball or another ball within one club-length, in the general area and no nearer the hole. The distinction worth checking is the condition in the ground, followed by where you are and whether the other requirements are met.