xxy = element

   xx  = what
     y = how often

    rw = read writes

    lc = library cache
    dc = dictionary cache

    de = data cache effectiveness
    ts = table scans
    sr = sorts memory/disk

    te = temp extent allocation
    rb = rollback extents
    ew = extent warning
    fs = free space
    me = max extent report

    vi = validate indexes

     m =  1 minute
     v =  5 minutes
     h = 60 minutes
     d =  1 day
     w =  1 week