PRESENT CONTINUOUS

It expresses a definite arrangement in the near future.

 "I'm taking an exam in October"  implies  that I have already entered for it.
 " Bob and Bill are meeting tonight"  implies that they have arranged it.

If there had been merely an expression of intention we would have used the Be going to form.

We can use the present continuous when we talk about arrangements ; an arrangement is a plan  but we are sure will happen. We often write it our diaries: such as the appointment with the dentist, meetings...

I´m going to the doctor's on Friday. (It's fixed. I have an appointment)

In these utterances the time must be mentioned or have been mentioned , as otherwise there may be confusion between present  and future.

TO TAKE INTO ACCOUNT:

1. There are some verbs that can't be used in the continuous. These verbs should be put into the future simple.

2. With verbs of movement  (arrive, fly... and especially "come" and "go") , verbs indicating position  (stay, remain..) and the verbs  "do" and   "have"  (food or drink) the present continuous  can express a decision or a plan without any definite arrangement.

Compare  PRESENT CONTINUOUS and  BE GOING TO.

I´m meeting my sister tonight = I have an arrangement with my sister.
I´m going to meet my sister tonight= my sister might get a surprise

Jean is going to get her hair cut . It is a plan, an intention. She doesn't know when exactly.
Jean is getting her hair cut next week. It is a fixed arrangement. She has an appointment at the hairdresser's