The expert mode extends the possibilities on a multiple records per page layout.
You can activate the expert mode in the settings of Multiple records per page (layout provided) option in export > multiple records tab (gear icon).
• You have a layout document with one or more pages, multiple records per page/spread that are not ordered/grouped in equal sets.
Comparing standard mode with expert mode
Standard mode needs equal sets of data – in the below example we have 8x the exact same copy of a business card (8 sets):
Expert mode has more flexibility. Instead of required set-copies you can add custom placeholders OUTSIDE of a copy.
In the below example we removed one copy and added an Index with the first and last name of the 7 business cards on the page .
Without expert mode turned on, the multiple records per page (layout provided) option would be unavailable.
Expert mode allows more freedom, but it also requires more attention in the setup. Be aware that detection of setup mistakes is reduced to allow more flexibility.
Setup mistake example:
In the above example you have 7 business cards and 7 index names in the top right corner  . If you would have accidentally added 8 index names but only 7 business cards, MyDataMerge would merge the seven business cards with records 1-7 and the eight index names with records 1-8. The new page would continue with record 9 then and there wouldn’t be a business card for record 8. Be aware to count the amount of placeholders you use in an expert mode setup like this .
This behaviour CAN be wanted, you just need to be aware of it.