Precise writers maintain a distinction between that and which:that is used as a relative pronoun in restrictive clauses
and which in nonrestrictive clauses.
In the book that is on the table is mine,
the clauses “that is on the table” is used to distinguish one particular book ( the one on the table ) from another or others
( which may be anywhere,but not on the table)
In the book , which is on the table,is mine,
the which clause is merely descriptive or incidental.
The more formal the level of language ,the more important
it is to preserve the distinction between the two relative
pronouns ;but in informal or colloquial usage,the words are
often used interchangeably.
I don’t understand that so and so.