What IIF is
IIF is QuickBooks Desktop's import and export format: tab-separated, with several tables interleaved in one file. A line starting with an exclamation mark defines the columns for a record type, and the data lines that follow begin with that type's keyword.
QuickBooks Desktop users, and the bookkeepers who have to move data into and out of it.
What Markdown is
A Markdown table is rows of pipe-separated cells with a dashed rule under the header. Alignment is set by placing colons in that rule.
README files, GitHub issues, documentation sites, Obsidian and Notion.
What changes when you convert IIF to Markdown
The largest table in the file is used, which is almost always the transactions. A typical export holds TRNS rows for the transactions and SPL rows for their split lines; those are two tables, not one, and combining them would produce a column layout that fits neither.
Cells are padded so the source lines up in a plain text editor, pipes inside values are escaped, and line breaks inside a cell become <br> because a Markdown table row cannot span lines.
What carries over from IIF to Markdown
One property of the IIF has no home in Markdown, and it is worth knowing which before you convert.
IIF distinguishes a number from the text of a number; Markdown does not, so every value arrives as text and has to be re-parsed by whatever reads it next.