Troubleshooting & messages · Free space and clean-up messages
What is "space this Mac has marked as reclaimable but has not released yet"?
Short answer: macOS has marked this space as reclaimable (purgeable) but hasn't released it; no app can force it. macOS hands it back automatically when space is needed.
Some of your disk is marked by macOS as reclaimable — space it can hand back whenever something needs it (often called purgeable). It hasn't actually been released yet, and no app can force it; macOS releases it on its own when space is needed.
That's why the free space shown by different apps can disagree. Storage Studio shows this amount separately so you know it's there.