Flexible constraints are tasks where we have the flexibility when it comes to the date of the project, when it's starting, and when it's ending. Usually ASAP (as soon as possible) and ALAP (As Late As Possible) tasks are flexible. An example of a flexible task is the development of the administrator interface of the website.
Inflexible constraints are those tasks that must start on and/or must finish on a certain date. An example of an inflexible constraint is the design of the website that must finish before the actual development starts. We have to abide by the exact start date and/or end date for inflexible constraints.