To import a BOM each information (Manufacturer, MPN) should be in its own cell.
BOM which contains multi line information in one cell cannot directly be imported.
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To be able to import such BOM, Excel can be used to split multiple lines in a cell into separate cells.
Open the BOM in Excel
Count the maximum number of lines in a cell (Manufacturers and MPN) which has to be split
Insert empty columns for the split cells accordingly
Select to column with multiple lines to be split
From Data Tab run Text to Columns.
Choose Delimited
Next step select only Other as delimiter and enter CTRL+J in the entry field.
Next step you may define a different destination for the cells.
Click Finish.