The S&P Global Composite PMI is a significant economic indicator that reflects the health of the manufacturing and services sectors. A decrease in the PMI, as reported for April, suggests a slowdown in economic activity. This can lead to increased market volatility and potentially impact investor sentiment towards assets like the SPDR S&P 500 ETF Trust (SPY), which tracks the performance of the S&P 500. However, the impact may be moderate as the PMI still indicates expansion (above 50) and investors may have anticipated this change.