Assume a system with 4 AMPs and a database with 1 GB of spool space. The per-AMP quota of spool space is 250 megabytes. If you set the spool space skew limit to 20%, any AMP is allowed a skew limit of 50 megabytes over the per-AMP quota of spool space. An AMP can use up to 300 megabytes of spool space if total spool space used does not exceed the 1 GB global limit for spool space.
MODIFY DATABASE d1 AS PERM = 1e9 SPOOL = 1e9 SKEW = 20 PERCENT;