Marriott's corp codes are usually 3 letter codes (hence easily guessable). Hilton's are usually numeric (think Starwood codes are numeric as well) & same corp code will have dif numbers depending on 'brand' - ie it's xxx number for full service Hilton, but yyy # for HGI,DT,ES. Also, sometimes the # for int'l Hilton is zzz.
All that, btw, is if you're booking online. If you're doing it via phone, you'd just ask what the (fill in the corp or promo) rate is.
Most corps/hotels (not specific to Hilton only) negotiate room rates based on volume per year. Some do it only w/ hotels near their offices; others across the chain if they're a large corp. If the corp hits the volume (or doesn't), it helps them (or not) negotiate for rates for the following year. Also the hotel (chain) gets x amount of revenue if corp folk do stay at their properties due to the rates, hence them doing the negotiation in the first place.
When I worked for F50 & F500 firms, they had corp rates with all the main hotel chains. I was never asked for id, although I could have produced it. Now that I'm an independent consultant, sometimes I'm told by the corp to use their corp rate (if they have one) since it lessens the expense report they'll have to pay. In this case I don't have the corp id, but have been given permission to use it (and presumably if challenged could get them to call).
On the Marriott forum, along w/ the codes, is the discussion of ethics & should/shouldn't you use ones you're entitled to. Obviously any hotel chain (and some FTers) will tell you no, never, you'll go to h*ll in a handbasket. Others are like who cares, it's there, you're helping the corp hit its #s. Others just buy popcorn & watch the FT debate
One caveat about using the code is if asked to produce id (which is rare), be prepared to pay rack rate if you can't produce it or are not a quick thinker as to why you wouldn't have it. The one that everyone seems to agree is asked is the gov't id. So unless you're a gov't employee (whether city, state, fed, etc) that one could be more problematic.
Also, when checking (if doing it via phone or view all rates), sometimes the publicly available rates are actually better than the corp rates, especially on the weekends. I normally ask what's the least-available rate, does the property have any specials (local) going. I got a great holiday rate one time in DC in Dec when I asked about least-expensive rate/any local promos; the rate for the convention I was attending was much higher. My client was thrilled I saved them $$ & I was thrilled I got hotel points/got to stay in a nicer hotel.
Re: what report Hilton sends, can't address a specific report but if you're using a corp code, my guess is their reservation system tracks that & so both sides know # volume of rooms used w/ that rate; let the negotiations begin for the next year.
Cheers.