This means that a cel can be reduced to only the portion of the animation which changes from the preceding cel. This requires less data than full redrawing if the static region is complex. For this reason it is advisable to minimize cel size as much as possible. The GIF89a specification includes all of the features from the GIF87 - including those which are no longer used. Some GIF89a editors let you set these properties anyway, but as with any GIF the aspect ratio and background color are better left alone.
Some editors also support the remaining cel-level options, which are just as well disregarded.
Any cel image may have its composition data interlaced . Each cel's control has an option to wait for user input before proceeding to the next cel. There is no convention as to what form this input should take, as no modern display applications support this feature.