Media Player DVD Movie Music CD - Unused Space on Writable CDs

Let's say you received a CD from your friend. He/she recorded some personal digital pictures to share with you. After viewing those nice pictures from the CD, you noticed that those digital pictures only uses less than half of the capacity of the CD. This is a writable (recordable) CD (CD-R). Now you are wondering if you can write more data onto the unused space of this CD.
The answer is yes, if your friend selected the recording option to allow next session when he/she was recording those pictures.
The answer is no, if your friend selected the recording option to disallow next session when he/she was recording those pictures. If you try to record more data to this CD, you will get a "no space" error.