Hello shaoguan,
that's an interesting question, though what I've heard for many of these up to 50,000 extra-rare characters is that there is no research consensus on their history or meaning, and often wild personal guesses abound instead. There may be a Chinese dictionary work that gathers the results of such research; someone else on the forums may know one. In any case, for a dictionary to be included in Pleco, it's got to have critical mass/to sell well, or it would have to be very expensive.
It's common for characters to be recorded in Pleco with pinyin only, without any indication of their meaning, because Pleco includes a Unicode dictionary with basic classifying information on each character included in the Unicode character set. I believe Pleco includes even more characters than what Unicode provides. You should be able to tell by the different font used for such characters in the dictionary listing.
Hope this helps,
Shun