is of a point (physical, decisional, etc.) beyond which one will proceed no further. An example would be a person who might agree to visit a bar with his friends, but will go no further (i.e., not partake in drinking alcohol).
The second meaning is that of a point beyond which, once the decision to go beyond it is made, the decision and its resulting consequences are permanently decided and irreversible. An example would be to commit funds to completing a project (as opposed to deferral or cancellation); once committed, the funds cannot be spent on another activity, and the project will either succeed or fail.