Transaction_subtype in FA_TRANSACTION_HEADERS

HI,

we have performed the Unplanned depreciaton for some assets through asset workbench.
The following inputs given while performing unplanned depreciation.

1. Type: Null
2. Amount: Depreciation Amount
3. Expense Account: Account

After that, I have checked the same assets in Fa_TRANSASCTION_HEADERS and column
transaction_subtype was dispalyed with Expense and NULL.

Please let me know, on which scenario transaction_subtype is displayed with EXPENSE.

Regards,
Madhu