You basically have 3 options in regards of variants with Flexify:
1. Show all variants
2. Show first available variant (it's determined from the first available one in Shopify)
3. Show cheapest available variant
You can limit the feed to include only one entry per Shopify product. More specifically, to include only the first product variant that is available for purchase. This is the default option.
Many customers prefer choosing the cheapest variant that is available for orders though.
How does this impact your feed?
E.g. if you have a product called "T-Shirt" with variants SMALL - MEDIUM - LARGE, then only ONE product will be included in your feed (T-Shirt) if you select the First Available Variant, respectively the Cheapest Available Variant.
If you select the all variants option then all variants will be included, i.e. 3 products: T-Shirt (SMALL), T-Shirt (MEDIUM), T-Shirt (LARGE).