When you are scoping the project, You decompose the project work into Work Packages using a WBS . After you have decomposed your WBS into individual Work Packages , Each one of the Work Packages contains a list of activities. For example, there is a work package that says "System Testing". This will be a work package that will be encompassing all the activities that a System Test Analyst is expected to do on the project .
You might either choose to in-source or outsource the System Test Analyst. There is a cost associated to that resource per hour based on the number of hours that the system Test Analyst will be expected to work on .So you roll up the costs for all activities and come up with a figure , lets say $30,000 For three months of work for the Test Analyst.
Then you can assign a contingency of lets say 10% or $3000
So the overall cost for that work-package will be $33,000 . This could be an individual Account or Line Item in your Finance System . Eg 2.2 - External Resource Now you may track the work being done and expenditure on this account through time sheets or other mechanisms and eventually your finance system will need to pay the invoice if it's external resource or you can pay for the resource from budget using an internal Departmental "Chargeback" mechanism.
THis is the Control Account Put simply , it's a line item with a bucket of Money which you can use to track your project expenditure for an External Resource
Mind you , a single Control account 2.2 External Resource can be used for any Number of Resources on Your Project who are External.You simply roll up all the costs and may end up with a control account of $100,000 with a Solution Architect and a Business Analyst as well.
Or you can have another Control Account for "Business Analysis" Or "Professional services from another vendor"
Your Finance Team should be able to guide you on how best to use Control accounts . From you as the PM, you only need to know that you need to track how much each of your resources is costing on the project
The resources may not be human. It can be a Software License as well
Individual Control accounts will add up to become your project budget You can apply contingency at 1) Activity Level 2) Work Package Level on the Control Account 3) Over all Budget
Before you know how much money , you need the Scope and you need the schedule. Those two need to be inputs into Cost Management Saving Changes...