What is the difference between Modernize Electronic Filing (MEF) and Legacy Electronic Filing?

http://www.irs.gov/efile/article/0,,id=204763,00.h...

The error messages are different.

The legacy program tended to parse the return one line at a time. An error early in the tax return, no matter how obvious, couldn't be fixed and you'd get just one error message. (Or worse, the IRS would just readd the totals and adjust the return and generate adjustment letters and work.)

0466 Form 2555 – Total Housing and Foreign Earned Income Exclusions (SEQ 1230) must equal the total of Housing Exclusion (SEQ 1140) plus Foreign Earned Income Exclusion (SEQ 1220).

This is a typical message. You'd have to be experienced in computer programming to know that the SEQ numbers simply refer to fields on the tax return.

MeF uses rule bursts and can compare different lines on the return for logical relationships.

F1040-017

If Form 1040, Line 8a 'TaxableInterestAmt' is greater than 1500, then it must be equal to Schedule B (Form 1040), Line 4 'CalculatedTotalTaxableIntAmt'.

vs. incredibly long ones:

F1040-016 If Form 1040, Line 64a 'EarnedIncomeCreditAmt' has a non-zero value, then [ in 'WagesNotShownSchedule' attached to Form 1040, Line 7, at least one

'WagesLit' must be equal to "HSH" and the corresponding amount 'WagesAmt' must be greater than zero ] or [ 'OtherIncomeTypeStatement' must be

attached to Form 1040, Line 21 ] or [ Form W-2 must be present ] or [ Form 1099-R must be present ] or [ Schedule C (Form 1040) must be present ] or [

Schedule E (Form 1040), Line 28A(b) 'PartnershipOrSCorpCd' must be equal to "P" for every entry in Line 28].

not the slightest idea, sorry