The financial statement elaborated for COSME-GA contains the same fields as the general one and it should be treated similarly (more information on how to complete a general financial statement can be found here). These main fields of the financial statement are the following:
"No contribution requested" is information coming from the signed grant, so this field is read-only.
The Certificate on the Financial Statement (CFS) must be provided, in the final periodic report, “for each beneficiary [and for each linked third party], if it requests a total contribution of EUR 325 000 or more, as reimbursement of actual costs and unit costs calculated on the basis of its usual cost accounting practices” (art. 20.4).
If a CFS needs to be provided, a warning is displayed in the application and a validation error appears if the user saves the data without providing the requested CFS.
All financial statements (each period has one financial statement) and adjustments (adjustment from a previous period for which you haven't declared all costs yet and the requested contribution) for the current periodic report are displayed here. Note: the button "add adjustment" is only available from period 2 (to adjust the costs requested for period 1).
The detailed costs of the selected financial statement are visible here. 
Note that the estimated budget for the action already supplied in annex 2 of the Grant Agreement doesn't appear here. You will have to introduce the actually incurred costs here.
The source document describing the costs is the Model Grant Agreement on the Funding & Tenders Portal.
Some cost categories have:
to complete the total amount of costs for that cost category
to click and complete the different amounts and corresponding explanations for the Use of Resources under a specific cost category 
to complete with explanations for only a part of the entered amount under a specific cost category. The following cost categories are included in the financial statement for the intermediary reporting:
a1 = Direct personnel costs
a2 = Direct personnel costs declared as unit costs*
b = Direct costs of subcontracting
d = Other direct costs
e = Indirect costs
f = Total estimated eligible costs
k = Maximum EU contribution
l = Requested EU contribution
z = Requested EU contribution eligible for CFS
* Please note that for projects with model grant agreement version 4.0 or newer, the "a2. Direct personnel costs declared as unit costs" category is displayed for beneficiaries who are either natural persons or SME owners.
The following cost categories are included only for the Financial Statement submitted in the final reporting period:
g = Income generated by the action (included only for the Financial Statement submitted in the final reporting period)
h = Financial contributions given by third parties to the beneficiaries (included only for the Financial Statement submitted in the final reporting period)
i = Receipts (included only for the Financial Statement submitted in the final reporting period and equal to the sum of the income generated by the action plus the financial contributions given by third parties to the beneficiaries;
