A demonstrative pronoun identifies a noun. The demonstrative pronouns in English are "this," "that," "these," and "those." The demonstrative pronouns are identical to demonstrative adjectives in both English and ASL, though, you use them differently.
In ASL, the gloss for the signed demonstrative pronoun/adjective above is generally identified as that rather than that/this, in which either THAT or THIS is the same in ASL. Remember that a gloss does not mean that it literally or directly represents an English word. It only helps shows an approximate representation of the sign.
In English, "this" and "these" refer to things that are nearby either in space or in time, while "that" and "those" refer to things that are farther away in space or time. In ASL, a location of the signed THAT in space is used to refer to things in space or in time.
that is why..
that ix very-good.
that is very good.
That is hers.
/\that++. or /\that IX.
That is the one.
Related tutorial: Determiners in sign language.