In fact, he is talking about C-type periodicals classified by unit. This classification is usually the standard set by each unit or school itself, and there has never been such a classification at the national level.
Because it is the standard of self-division, the standard is naturally different for different units. The author must publish a paper based on the requirements of his own unit. For example, in some schools, Class A refers to the journals included in SCI, Class B refers to the journals included in EI, Class C refers to the Guide Core and North Core, and Class D refers to the academic journals officially published at home and abroad except ABC. So if the author wants to publish Class C, he actually wants to publish core journals. If it is class D, this is also the most commonly posted. After all, the core is very difficult.
Of course, due to the different classification standards of each unit, the D category of this unit is a general publication, and in that unit, the general publication may be classified as E category. Therefore, when submitting a manuscript for consultation, we must be clear about the classification standard of periodicals in our unit, so as to correctly select periodicals.