"Miele offers four different sizes of vacuum bags in the market, including GN, FJM, U, and KK. To make it easier to identify which type of bag is suitable for your vacuum, Miele color-coded them for convenience.

When it comes to Miele canister vacuums, they can use either FJM (Red tab) or GN dustbags (Blue tab). 

For Miele upright vacuums, U dustbags are the appropriate choice, while KK bags are compatible with Miele upright/stick vacuums.

It's common to become confused when searching for Miele canister vacuum bags, particularly with the GN (Blue tab) and FJM bags (Red tab), which are quite similar. The only distinguishing feature between these bags is their size and tab colour.

Miele GN vacuum bags are designed for larger canister vacuums like the Classic C1 and Complete C2 & C3 Series, and have a capacity of 4.76 quarts.

In contrast, Miele FJM vacuum bags are smaller and have a 3.7 quart capacity. These bags are intended for smaller canister vacuums like the Complete C1, Compact C1, and C2 Series.

Despite their size differences, all of these bags have the same fitting. So, if you mistakenly purchase the wrong ones, the only variation will be how much dust and debris each bag can hold."

Source: https://www.yourspares.co.uk/advice/which-miele-vacuum-bag-do-i-need.aspx