Can someone explain the differences between 'memorize' and 'save to Library'?

Is there a reason to use one rather than the other?

Memorize makes the current settings of a figure be the default, but it doesn't save the figure anywhere. It's mainly used by people who are setting up their own Poser content. Save to library will, of course, result in the figure being added to the library for future use.

I mostly use memorize for temporary activities and saving to the library for more complete efforts. Memorize is a fast save and saving to the library involves writing a new file and takes a little longer. It's that time difference that makes memorize more useful. Saving to library saves all figure and prop information so that it loads everything the next time Poser is started. Memorize only saves parameters for that session of Poser and it's lost after that. Memorized settings can be activated with keyboard shortcuts opening a library items loads something new into the image.

The most powerful feature for memorizing is that saving to the library will save the memorized settings as default settings. Let's say that you've morphed a figure exactly the way you want and that figure will be used over and over. Saving that figure to the library will save the figure with all the morphs, but will not save that figure to a default state with the morphs. Using a restore action on that figure will revert to the figure's default factory loaded state.  By memorizing the figure first, and then saving to library, a restore will only reset changes made since the figure was loaded, but will not get rid of the morphs that had been saved. Resized/repositioned props, hair, imported files, etc. will retain whatever changes had been made to the saved version. This is very useful for a figure that has a lot of resized, repositioned, and parented props.

There are a few other areas where I use memorize. 

• Morphing and posing a figure. Memorizing a morphed figure saves that morph as the default when "restore" is used. When I'm doing things that are going the wrong way and it's no longer easy to get back to an earlier state,  I'll hit SHIFT-CTRL-F to reset the figure back to it's default state. Without using memorize first, that default will be the default figure's appearance and not the version that I morphed to.

• Setting a hair prop element to stay with a morphed figure. If a restore operation is made to a figure that is using hair, that has been resized or repositioned to fit on the figure, the parented hair also gets restored to its default state and no longer fit or even be on the figure that it was modifed to fit. Memorizing the hair fixes that problem. Performing a restore on the figure doesn't move the hair anymore.

• Temporary lighting or camera settings. If I'm kind of happy with the lighting or camera angle I've picked, but not quite ready to save to the library, memorize will let me store those settings so that I can hit some keyboard shortcuts to quickly get back to them. Most of the time, I'll use camera dots for that purpose, but there aren't any memory dots for lights.

:blushing: Well you learn something new everyday LOL In 3 versions of Poser I have never noticed or used the memorize funtion, sounds very handy too.

Thanks for bringing it up.

Memorizing is especially useful for short GIF-type animations where you need to repeat
or loop an action.   Start at point A; memorize all; move forward through the frames
to your point B; then go to the last frame and restore all.

Also, memorizing a figure pose is an essential step in creating conforming clothes (after rigging).
Without it, conforming produces strange results - sleeves and body parts twisting in strangest directions.

Thank you all, prompt and informative as usual.

If the lights are good in a particular frame,  I'll memorize them, go back to frame 1 and do a restore, then erase any other light moves on the timeline. This makes the lighting consistent across all frames. I find it very useful.