what should i call this?

i've been working on a new language recently (still unnamed) and i can't
figure out what to call a certain feature of the langauge... i guess the
easiest way for me to explain it is to give an example, so here goes:
ei = is
sān = this
vonh' = book
eica sān vonh'.
"this is a book."
ei sān vonh'.
"this is the book." or "this is what a book is."/"a book is this."
basically "eica X Y" means X is Y, but Y may or may not be X, whereas "ei X Y"
means X is Y and Y is X...
this "-ca" can be used on other verbs also... not just "ei"... any ideas?

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