Alarm levels, 1st, 2nd, 3rd, etc. were originally established to dispatch a predetermined number of specific companies to a specific location, and to make predetermined relocations, all by sending one telegraph signal. Each additional alarm essentially multiplied the 1st alarm response (sometimes slightly more). The anecdotal correlation was that the number of alarms transmitted reflected the size of the incident, usually a reasonable assumption, but really, only a reflection of the number of units sent.
With the variable number of 1st alarm units, as changed over the years and by areas of the city and by policy revisions and by building specific responses (10-76, 10-77, etc), automatic 2nd alarms, and 10-75 revisions, even the number of units initially sent varies significantly. By total number of units sent, a Midtown 10-77 is a 3rd alarm in residential Queens. Without predetermined response orders or relocations, each incident at a particular box location may differ, from one incident to the next. Comparing 6 and 7 firefighter staffed companies from past decades to today's responses is difficult as they have changed so much.