for saving data to custom fields i only have the option of saving the description of movie in custom fields
For a plugin, yes. That is by design. Plugins save data only to standard fields. For the most part, the meaning of each standard field is clear enough it should be the same regardless of the source. So what most users want to do, for example, is record
Genre from the best source—or either source that has it—not
Genre–Source1 and
Genre–Source2.
Description, on the other hand, will vary considerably between sources, so the program offers a way to save them separately.
Many sites will have data completely different from the standard fields provided. This data may be retrieved using a script and saved in custom fields. So, if for some reason you wanted to record something like
Genre–Source2, that is how you would do it.
i cant save, say, cast of movie according to different sources in different fields.
Maybe you just picked a poor example, but credits and awards data are things you should
not attempt to get from more than one source. This will dramatically increase the likelihood of error in recording the relationships between people and awards and movies. You may still, however, save such data from a secondary source in a separate custom field using a script. For example, I record the
AllMovie "Production Credits" page in a memo field—just in case I have some question about the data from IMDb.