If you’ve found a fractal shape that you like, you can “lock it in” by fixing the seed and then playing with the other parameters in order to generate the final image that you’re looking for.  You will also  note that the seed itself is accessible and changeable from the Modify Parameters menu though you should rarely need to access it from there.

If the seed is fixed, there will be a check mark next to the menu item.  When the item is unchecked, each time that a new render is initiated a new random number seed is used, generating a unique fractal.  Think of it like rolling the dice!

If the seed is not fixed and you accidentally generate a new seed, the preceding seed can be retrieved by the following method.  First, select the Undo command before you do anything else.  Now, immediately fix the seed!  This will  bring back the seed used for the image prior to the one displayed currently.  Note that this will only work this way if Fixed Seed is off.  If you do not fix the seed after selecting Undo, then you will merely regenerate the currently showing image and you will no longer be able to retrieve the prior seed. 

Note that even though you have fixed the seed, there are some other parameters that can drastically affect the final image.  The most important of these is the Noise Type.  As long as the noise type is not changed, fixing the seed will cause subsequent renders to generate an image with the same “basic” shape.  Changing the Noise Type, however, will result in a completely different shape even if the seed is fixed.  Changing the Noise Type back will of course restore the original shape.