REST URI convention - Singular or plural name of resource while creating it

For me is better to have a schema that you can map directly to code (easy to automate), mainly because code is what is going to be at both ends.

GET  /orders          <---> orders 
POST /orders          <---> orders.push(data)
GET  /orders/1        <---> orders[1]
PUT  /orders/1        <---> orders[1] = data
GET  /orders/1/lines  <---> orders[1].lines
POST /orders/1/lines  <---> orders[1].lines.push(data) 

The premise of using /resources is that it is representing "all" resources. If you do a GET /resources, you will likely return the entire collection. By POSTing to /resources, you are adding to the collection.

However, the individual resources are available at /resource. If you do a GET /resource, you will likely error, as this request doesn't make any sense, whereas /resource/123 makes perfect sense.

Using /resource instead of /resources is similar to how you would do this if you were working with, say, a file system and a collection of files and /resource is the "directory" with the individual 123, 456 files in it.

Neither way is right or wrong, go with what you like best.

I don't see the point in doing this either and I think it is not the best URI design. As a user of a RESTful service I'd expect the list resource to have the same name no matter whether I access the list or specific resource 'in' the list. You should use the same identifiers no matter whether you want use the list resource or a specific resource.