Cost category : EUC1-ETDataAccess-Bytes ?

It stands for "ElasticThroughputAccessBytes", also known as "Elastic Throughput Requests - Data and Metadata Reads and Writes" summed up.

If you filter EUC1-ETDataAccess-Bytes in Cost Explorer under the Usage Type filter, and they when looking at your graph, you Group By API Operation - does this provide any additional information as to what this usage may be referring to?

Alternatively, for quick access to this information, you could open your Billing Dashboard (Bills page of the billing console), look at the relevant month of your bill, find Batch service, expand the relevant region (in this case eu-central-1, which must be Frankfurt), and you should find more detailed description for the line item referring to EUC1-ETDataAccess-Bytes.

But also - check out this - https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/using-ppslong.html - the bulk API offer file :) You can pretty much access info about pricing and information about usage types and respective description for any service. This is something like an extended version of pricing page, in API-friendly format. However, there's no pricing API for Batch, really... just because Batch is free on its own. You only pay for underlying resources (like EC2 instances... Lambda functions, etc) that you create to store and run your application.