There are tiers and each message represents a 10% increase.

'No Activity': 0 - 199
'Indistinct Activity': 200-399
'Faint Activity': 400-599
'Slight Activity': 600-799
'Modest Activity': 800-999
'Distinct Activity': 1000-1199
'Robust Activity': 1200-1399
'Vigorous Activity': 1400-1599
'Intense Activity': 1600-1799
'Extreme Activity': 1800-1999
'Bursting Activity': 2000+ (Complete)