There are several options when it comes to deciding where should the money come from. The send statement makes it handy by providing the following possibilities:

​
Single source

The simplest way of sending a monetary value is from a single source. Here, we draw COIN 100 from the world account: 
send [COIN 100] (
  source = @world
  destination = @users:001
)

​
Ordered sources

Using an ordered source block, you can defined several accounts to draw from sequentially until the desired monetary value is reached. 
send [COIN 100] (
  source = {
    @users:001:wallet
    @payments:001
  }
  destination = @orders:001
)
In example above, if the balance of COIN on the account users:001:wallet is 30, another 70 will be drawn from the payments:001 account. Ordered sources can also be maxed to a monetary, preventing them from being drawn more than the amount specified: 
send [COIN 100] (
  source = {
    max [COIN 10] from @users:001:wallet
    @payments:001
  }
  destination = @orders:001
)

​
Portioned sources

In addition to sequential accounts, source blocks can also use fractions to split the expense onto multiple accounts. In any case, the summed total of fractions in a block needs to be equal to 1 and the remaining keyword can be used to reach that total. 
send [COIN 100] (
  source = {
    10/100 from @platform:marketing
    remaining from @users:001:wallet
  }
  destination = @orders:001
)
Out of convenience, percentage notation is also available: 
send [COIN 100] (
  source = {
    10% from @platform:marketing
    remaining from @users:001:wallet
  }
  destination = @orders:001
)

​
Nested sources

Source blocks can be nested with a combination of recursive ordered / portioned specifications: 
send [COIN 100] (
  source = {
    50% from {
      max [COIN 10] from @users:001:wallet
      @users:001:chest
    }
    remaining from @payments:001
  }
  destination = @orders:001
)