It's a "letter of intent".
This is literally saying "we intend to buy 100"...

If you want to know how strong a letter of intent is, I would reference this:
Trans States Holdings announced the signing of a letter of intent on 2 October for 50 firm orders plus 50 options for MRJ aircraft, spread across the 70-seat MRJ70, 90-seat MRJ90 and the proposed 100-seat MRJ100.

Although no delivery schedule has been set, TSH vice-president of scheduling Fred Oxley told Flightglobal earlier this month that the first quarter of 2014 is the target to allow the MRJs to replace aircraft operating under existing capacity purchase agreements between regionals and US majors.


.... article is from 2009.
I mean to be fair, TSH still has the options, but the damn airplane is too heavy to ever fly anywhere thanks to scope clause. When they finally start delivering TSH will probably just sell their delivery positions to some Euro carriers that don't have the scope restrictions that we do in the USA.