The flag TraceMarkSweep logs the number for the phase in the mark sweep code. This is pure debugging code that does not give much information:

$ java -XX:+UseSerialGC -XX:+TraceMarkSweep SysGC 10
 1234 1234 1234 1234 1234 1234 1234 1234 1234 1234

This does not help much even if combined with PrintGC:

#1: [Full GC (System.gc()) 1234 310K->310K(497024K), 0,0125369 secs]

or PrintGCDetails:

#1: [Full GC (System.gc()) #1: [Tenured 1234: 310K->310K(342720K), 0,0087471 secs] 310K->310K(497024K), [Metaspace: 2832K->2832K(1056768K)], 0,0088441 secs] [Times: user=0,00 sys=0,00 real=0,01 secs]

There are already trace events for the phases that include information about the time for the phase and a more readable name.