First of all, according to the previous answer, it seems that you have an idea about the classification table.
Then let me guess.
In the classification table you give now, you should see a point between all the contents and codes, and the number of this point indicates the level of the corresponding item.
The F04C 18/077 you listed is a three-level entry, with F04C 18/063 at the upper level, F04C 18/02 at the lower level and F04C 18/00 at the lower level.
F0 1C 18/077 and F04C 18/067~073 are juxtaposed, as are fuel oil and 18/04 and 18/06.
As for how to interpret F04C 18/077, it is "F04C 18/00 rotary piston pump specially suitable for elastic fluid (F04c19/00 with liquid ring or similar parts; The working fluid in the rotary piston pump is discharged only through 1 one or more reciprocating pistons (F04B) ".
F04C 18/02 ... arc meshing, that is, each mating element has arc transmission motion, and each element has the same number of teeth or tooth equivalents.
"F04C 18/06 ... is different from the following inner shaft type.
"F04C 18/077 ... with gear drive"
Specifically, it is a kind of rotary piston pump with gear transmission device, which is different from the internal shaft type and circular arc meshing type, and is especially suitable for elastic fluids. The last keyword is "rotary piston pump"