From this page:
Common paint tool settings in the Paint panel
- The duration of the paint stroke. Constant applies the stroke from the current frame to the end of the duration of the layer. Single Frame applies the stroke to the current frame only. Custom applies the stroke to the specified number of frames, beginning with the current frame. Write On applies the stroke from the current frame to the end of the duration of the layer and animates the End property of the stroke to match the motion with which the stroke was drawn.
I would suggest reading more on that page. Also, you'll probably find this page helpful as well.