French to English Bus/Financial Law: Taxation & Customs tax warehouse / tax declations

So I did look and see that it refers to ten-day periods, ten whatever periods, but different examples are given in the text.

The context is: the declaration of products for tax purposes.

The tax office is inspecting the declarations and commenting on them and referring to various "décade".

Examples in the text: décade is a heading for a column with number entries 1,2,3,4 and so on, and then other times the décade entries are years 2017, 2018 (with periods within the years set out under a different column).

thanks