What does 'separate' in sequelize mean?

Additional to @robertklep response:

As you know now it separates an otherwise joined query.

This means that it would be more performant in some situations where you have many joins and nested joins (can have a huge impact in some). Nested joins make Sequelize take more time in a single big query than running multiple small ones. The problem is pointed out as a deduplication operation:

See here: Slow associations in SequelizeJS

I found this in the current code:

If true, runs a separate query to fetch the associated instances, only supported for hasMany associations

To elaborate: by default, to retrieve the related model instance, Sequelize will use a SQL JOIN. By enabling separate, Sequelize will perform a separate query for each of the associated models, and join the resulting documents in code (instead of letting the database perform the join).

Assume I have Product model with a hasMany association to the Tag model ("a product can have many tags associated with it").

Here's the "regular" query:

SELECT
  `product`.`id`,
  `product`.`title`,
  `tags`.`id` AS `tags.id`,
  `tags`.`name` AS `tags.name`,
  `tags`.`productId` AS `tags.productId`
FROM `products` AS `product`
LEFT OUTER JOIN `tags` AS `tags`
ON 
  `product`.`id` = `tags`.`productId`;

Here are the separate : true queries:

SELECT 
  `product`.`id`,
  `product`.`title`
FROM `products` AS `product`;

SELECT
  `id`,
  `name`,
  `productId`
FROM `tags` AS `tag`
WHERE 
  `tag`.`productId` IN (1);