|Specifics: Time-coded time-lapse, BW stock motion comparison|
Stock footage refers to a sequence in film or television that can be frequently re-used, occasionally in multiple different productions. It is commonly used in animation to save the time and expense of having to re-draw sequences that are more or less assured to take place in every single episode.
In the context of Transformers, it applies to most of Japanese-made anime series: The Headmasters, Masterforce, Victory, Beast Wars II, Beast Wars Neo, Robots in Disguise, and the Unicron Trilogy shows. All of those series would almost ritualistically showcase the same clips of characters transforming, combining, or deploying unique weapons. These sequences often seemingly transport the characters to a setting different from the actual events of that part of the episode, as the entire standard background fades away and is replaced by streaking speedlines and flashes of color, so they can be used no matter where the characters actually are in the script.
Stock footage sequences typically have more detail, higher frame rates, and more exaggerated, prolonged posturing movements than the standard storytelling animation surrounding them. Supposedly, this is the method in which Japanese marketing has decided a toy's gimmicks are best "highlighted" in these advertoons.
The dub for Transformers Cybertron often poked fun at the practice, having characters narrate their filler stock-footage in ways that implied boredom or curiosity ("Why do we always go this way?"), or simply filled excessively-long sequences with snappy patter among the characters involved. Of course, Cybertron was particularly egregious in its dependence on using stock footage to fill time like the visual equivalent of styrofoam peanuts.
This practice is different from the stock animations used in the CGI-animated Beast Wars show, which often used "standard" sets of model movements for the characters' transformations from beast to robot mode to save the animators' time. However, these animations were not stock footage; each sequence had to be rendered out normally as any other each time they occurred, using different camera angles with the characters in the same scenery as they were before and after they transformed. Also, it stands perfectly well to reason that the various characters would transform the same way each time.
In the Transformers Animated series, most characters have stock footage for their transformation sequences, but so far they have usually only been used once apiece, with nearly all transformations portrayed in "standard" animation. The series' opening credits shows a great deal of stock footage that has rarely, or never, appeared in the actual cartoon.
The original Generation One cartoon was not immune from the use of stock footage, however its use was more limited and was generally used in a similar manner to Beast Wars, with pre-canned transformations employed over different backgrounds depending on the situation, without any cutscenes. Not surprisingly, the most notable of these is a transformation sequence for Optimus Prime which was used in both directions (truck to robot, and robot to truck), however others can be found, primarily throughout season 1, due to the smaller cast.