The example of biology in the instructions is awesome.
A Missing Rem that represents a concept or something in the real world. What means “A missing Rem”?
Where I can find more examples or articles for better understanding them?
Additionally, Missing or Deleted Rem can also happen when the target of the reference (the internal blue links) got moved or deleted, or whatnot.
Don’t forget that you can indent things. So under a concept you can nest other concepts.
You can think of Concepts as the key terms (you may find them in your book as terms with special formatting, or terms that are given a definition) and descriptors as characteristics of those terms or additional information regarding them.
It may be easier for you to understand, if you specify the fields you are studying or give some textbook examples (or links to internet pages that say the same thing, if your textbooks aren’t in English). We can take notes on them so you can compare a live example to your own source material. 
“Missing Rem” I think that occurs when wifi gets slow and can’t load the rems shown (at least that’s what happens to me). If you want some more examples of biology concept-descriptor notes, here’s a link to what I have shared currently. Link
Many thanks. I like your sample of "Missing Rem’. Maybe it will happen to me some day while the storage file of RemNote gets big.
Sort knowledge by “concept” and “descriptor”, it is a new angle for me to note down knowledge from the textbook. I am struggling to distinguish the “concept” and “descriptor” during the reading.
Glossary in the textbook is concept without doubt, however, some explanation can be concept or descriptor. Besides the way the instruction book gives, is there any tips to help classify the content into concept or descriptor?