need clarification deprn_source_code in FA_DEPRN_DETAIL table

Looking for clarification on below questions

1)What does the BOOKS, DEPRN and TRACK values mean in the DEPRN_SOURCE_CODE field on the fa_deprn_detail table? and under what circumstances are records created with these deprn_source_code.

2) If the rows with a source code of ‘B’ mean that depreciation has not yet run on the asset, why is the DEPRN_RUN_DATE populated for these records? What does thisdate signify?