X-propagation describes how unknown values move through digital logic and affect functional correctness. X-values may come from uninitialized storage elements, incomplete reset structures, RAM contents, conflicting drivers, unused branches, don’t-care assignments, black-box boundaries, or unimplemented logic. Some X-values are harmless because they remain isolated or inactive. Others become serious risks when they reach primary outputs, state machines, clocks, resets, low-power controls, or other functionally visible targets. RTL simulation and synthesis interpret X-values differently. In simulation, X is a visible unknown value. In synthesis, X may be treated as a don’t-care condition that allows logic to be optimized. This semantic gap can create RTL-to-gate mismatches. X-optimism may hide a real issue by converting an unknown condition into a known result. X-pessimism may create excessive unknowns that do not represent actual silicon behavior. Both effects reduce the quality of verification results and make debug decisions less reliable. Different X-propagation modes provide different trade-offs. Standard RTL behavior may be fast and familiar but can mask uncertainty in condit...