I think the best way to understand in detail how the formalism (Minimal Recursion Semantics) works is simply to have a look at the original paper by Copestake et.al. You may want to skip the motivation and formal definitions and focus on the representation and the implementation into feature structures/HPSG.
Maybe this summary helps for an overview, although it focusses more on the semantic underspecification mechanism and not on the representation in a feature structure.
The naming of the variables follows a specific pattern. Each index is a combination of a letter and a number (continuously numbered; numbering independent between different letters). The letters are abbreviations for certain types (this is already explained in the tutorial you linked to in the answer you gave yourself):
- x is an indivdiual argument in an elementary predication (EP). An elementary predication consists of a relation symbol with its associated arguments, e.g.
see(x1, x2) ("x1 sees x2"), where see is the relation and x1 and x2 are arguments. An individual may be e.g. Peter, or the pen which lies on my desk. A difference is made between scopal and non-scopal arguments.
- e stands for for eventuality. An eventuality may be the event see.
- i is a generalization over eventualities and individuals.
- h stands for handle and is used for handles and labels. A handle is a tag which links up a scopal argument position with the (conjunctions of) EPs that fills it, and a label identifies an EP as belonging to a particular tree node. E.g., we could give the EP
see(x1, x2) the handle
h1: see(x1, x2) and later reference that EP inside another EP by using its handle; in this usage the handle is called a label.
The actual magic about these variables happens in how they are used together (coindexation: If two slots are filled/indexed by the same variale, then this means that the same thing (which is denoted by that variable) fills in both of the roles indicated by the two different slots), and (peculiar to this particular framework, not a universal thing in feature structures) in the equality constraints listed under
HCONS (HCONS = handle constraints; qeq = equality modulo quantifiers).
Let's go through the example you linked step by step:
The top handle, i.e. the head of the entire thing is
h0. The first equality constraint in the
HCONS list states that
h0 is qeq-equal to
h1. So the top of the entire sentence that is denoted by the variable
h0 is equal to the thing denoted by the label
h1. Now what is the thing denoted by
h1? It's precisely the element which has the value
h1 in its
LBL (label) feature. This is the relation
是_v_cop⟨2:3⟩, i.e., via conindexation between the two uses of the variable
h1 together with the equality between the variables
h1, the top of the entire tree is the verb - an observation which matches up with the dependency analysis (the thing with the arrows) given in the lower picture (here, the concepts top/head/root are roughly the same).
The verb, which we briefly reference by its handle
h1, has three arguments: An eventuality argument
e2, and two individual arguments
x8. So let's track these variables:
e2 is used as the index (more precisely:
e2 is the value to the attribute
INDEX in the feature structure) of the whole thing. The index corresponds to a distinguished normal (non-handle) variable. So the variable for the eventuality which is denoted by the verb serves as the variable for the eventuality denoted by the entire structure, which makes sense because the core meaning of the sentence lies in the event that the verb expresses.
x3 is also used (coindexed) as the
pronoun_q. So coindexation of variables tells us that whichever individual is the first argument (presumably the subject) of the verb is the same that is referred to by the ordinary pronoun and the question pronoun (I suppose that's what the abbreviations mean).
A similar reasoning applies to the variable
x8 which is referenced in the relations
Let's now look at the existential quantification, i.e. the relation
exist_q⟨-1:-1⟩. In the elementary predication There is a pig which snores (or A pig snores or Some pig snores, all treated equivalently), the scopal argument pig is the restriction and the scopal argument snore is the body (or nuclear scope) of the quantifier there exists. In the predication All happy children laugh, happy children is the restriction and laugh the body of the quantifier
all. The restriction of this particular existential quantifier in use has the label
h14. This variable serves as a placeholder for whatever is to be inserted as the restriction of the quantication. The handle constraints in
HCONS tell us that the variable
h14 is quantifier-equal to the relation denoted by the label
h9. If we now search for this label among the list of relations, we find that it is the handle to the predication
card⟨4:5⟩. So the predication
card⟨4:5⟩ fills in the role of being the restriction of the existential quantification predication
Unfortunately I don't speak any Chinese so I can't give an explanation with reference to the precise meaning, but I hope you get the idea. It's pretty much like a paper chase: You start tracking where else the variable you are interested in is being used, in addition (particular to the MRS framework) follow the path through the equalities that link different variables together, and the semantics of each variable is precisely the combination of places where it is used to fill a particular position in the feature structure. This is the crucial point of coindexation, which is one of the key ideas of feature-based grammars.