Web API: Need to understand a web api attribute routing

They both are different,

customers/{customerId}/orders

customers/{customerId}/orders/{orderId}

Sample Scenario:

You need to check all teh items that you have ordered in Amazon or Flipkart site

To view all the orders placed by user then use
customers/{customerId}/orders

Which will return the collection of orders, so resposne is - IEnumerable

When you have to check detail about single Item you ordered last, say a mobile phone which has oredr Id 24,

Then use the other URL

customers/{customerId}/orders/{orderId}

customers/{customerId}/orders/24

Which will return detail of that particular order, with full infomation, so response will be Order

That is why both have different return type [Response] and URL is different.

Mark as answer, if helpful.