Three layers
On this page11
The primary receptacle
The thing the sample is in. A tube, a vial, a bottle, a cup. This is the layer a container catalog can supply and the only one we do.
The secondary packaging
A leakproof enclosure around the primary receptacle. Its job is to contain the sample if the primary fails, which means it is a containment device rather than a wrapper.
The absorbent, between the two
"Absorbent material must be placed between the primary receptacle and secondary packaging" and "must be of sufficient quantity to absorb the entire contents of the primary receptacles."
Read entire contents literally
Not enough to mop up a dribble. Enough that if every primary receptacle in the parcel emptied, the absorbent holds all of it. That is a quantity you calculate from the volumes you are shipping.
Which is the step most often done by eye
A pad in the bag looks like compliance. Whether it absorbs 4 millilitres or 40 is a specification, and with multiple primaries in one parcel the total is what counts.
The rigid outer packaging
Rigid is the word. It carries the markings, it takes the handling, and it is what the drop test is performed on.
Why three and not two
Because the failure modes are different. The primary fails by leaking, the secondary catches it, and the outer survives being dropped, crushed and stacked without either of the inner layers being exposed.
An itemised list of contents goes between the layers
Which is part of the system rather than a courtesy. Somebody opening a damaged parcel needs to know what was in it before they find out.
The system is tested as a system
Which is why buying the three parts separately from three suppliers and assembling them is not the same as buying a tested packaging. The test result belongs to the assembly.
So for Category B, buy the kit
From a supplier who sells and certifies the complete packaging. We can tell you what the inner layer has to do; we cannot sell you the thing the regulation describes.