what determines if Form 8453 must be filed for capital gain transactions ?

[RustyShackleford]

My wife's brokerage account has sales too numerous to enter by hand, and importing doesn't seem to work right (problem at broker's end, we think) so I simply enter summaries on Form 8949.

Last year, Turbotax told me I needed to send in Form 8453, with the brokerage 1099-B (and maybe related Forms 8949).

This year, it's not doing so.   I am aware of the excellent answer here (https://ttlc.intuit.com/community/taxes/discussion/i-e-filed-my-return-the-filing-instructions-tell-...) but this doesn't seem to explain the difference from last year.   There most definitely ARE wash sale adjustments this year, but only on type A and D transactions; the few type E transactions are explicitly listed.

[Mike9241]

The IRS does not need details of transactions that are properly reported on lines 1a and/or  8a of Schedule D (See exceptions 1 and 2 starting on page 3  of 8949 instructions - link below). These are 1099-B transactions for which proceeds and tax basis are reported to the IRS without adjustments, such as wash sales. All other transactions are reported on Form 8949. If each transaction is detailed on the 8949, nothing further needs to be submitted to the IRS. However, summary totals for each type (A,B,C,D,E,F,X as shown on the 1099-B) may be entered on the 8949. Code M must be entered on lines that report multiple transactions. In this case, the 1099-B or substitute must be submitted to the IRS using Form 8453 - see instructions. It's possible you don't see Form 8453 because you didn't enter code M. 

https://www.irs.gov/pub/irs-pdf/f8453.pdf 

https://www.irs.gov/pub/irs-pdf/i8949.pdf 

[RustyShackleford]

In column (f) in the 8949s is "MW".

[RustyShackleford]

It also says "see attached statement" (on copies 'A' and 'D' of the 8949).   But (unlike last year), it doesn't include Form 8453 in the printed return, nor do the instructions tell you to mail it in.  

[RustyShackleford]

Mystery solved, I think.   After I filed my return, it THEN came and said "there are add'l forms you need to file", and this was the 8453.