The LaTeX (2e) source code is quiet hard to read. There are a lot of macros with
@ symbol. I understand those are "latex macros".
There is, in particular, a series of macros called
\thr@@ (and maybe more). Their
\char"3. I wonder (1.) how and where those are defined?
They seem to be used mainly to identify internal registers. "The Texbook" mentions that there are 256 internal registers of each type, e.g.
\box256. So which register does
After reading Martin's answer I reached to the texbook, page 121:
\newcount, plain TEX provides
\newbox; there also are
\newinsert, for features we haven’t discussed yet. Appendices B and E contain several examples of the proper use of allocation. In the cases of
\newread, etc., the allocated number is deﬁned by
\chardef. For example, if the command "
\newbox\abstract" is used to deﬁne a box register that will contain an abstract, and if the
\newboxoperation decides to allocate
\box45for this purpose, then it deﬁnes the meaning of
\abstractby saying "
\chardef\abstract=45". TEX allows
\chardef’d quantities to be used as integers, so that you can say
\copy\abstract, etc. (There is no
\box\@ne is equal to