While a fermata instructs the musician to pause the action of playing, the Freeze Melody Mark instructs the musician to pause the decay of the sound itself.
Young conductors often mistake the Freeze Melody Mark for a long fermata. This is a grave error. A fermata builds tension through the physical effort of holding a bow or sustaining a breath. The Freeze Melody Mark releases all physical effort, replacing it with pure psychological will. To play it wrong—to sustain the note physically—is to create a boring, long tone. To play it correctly is to create a miracle of collective hallucination. freeze melody marks
A is different. It is an instant glacier. While a fermata instructs the musician to pause