Legalization
Beginner
Snapping every part into a proper slot in a row so nothing overlaps, while moving each part as little as possible.
Novice
The step after global placement that moves each cell to a free slot (site) in a row, so nothing overlaps and nothing sits in a forbidden area, while moving each cell as little as possible.
Expert
Greedy (Tetris), row-based dynamic programming (Abacus), flow-based, diffusion-based and negotiation-based methods. It must also respect fences, padding, edge-spacing rules and multi-height rail alignment. Large displacement here undoes the wirelength and timing that global placement achieved.