Which variables should be described in Spreadsheet?

It depends on the type and schema of the dataset.

For example, in the scheme of "Cost Data" type:

• Required fields: ga: date, ga: medium, ga: source;
• Required at least one of these fields: ga: adCost, ga: adClicks, ga: impressions;
• Additionally, you can add any variations in the following fields: ga: adFinalUrl, ga: creativeLine2, ga: creativeLine3, ga: referralPath, ga: adGroup, ga: adwordsCampaignID, ga: adwordsCriteriaID, ga: keyword, ga: campaign, ga: adDisplayUrl, ga: adSlotPosition, ga: adSlot, ga: adContent, ga: adDestinationUrl, ga: adMatchedQuery

More information about types of datasets can be found in Google Help.