A tanzen obi is a small, lightweight belt used to tie together a tanzen (thick, padded jacket), haori (kimono jacket), and a kimono.
These are also oftentimes used in places like an onsen (public bath) or ryokan (Japanese-style inn), typically with a yukata. These should not be worn formally and should only be used indoors. A heko obi or, better yet, a kaku obi should be worn by men outdoors.
Due to their size and oftentimes being fairly malleable, tanzen obis are sometimes used as a datejime (a strap underneath an obi to hold up the kimono).

Medium Size (as indicated on the package, closer to US/EU small)
Datejime:
Length: 85cm (33.5")
Tanzen Obi:
Length: 90cm (35")

Age:
1990s

Quality:
Rayon 40%, Nylon 30%, Rubber 30%
S~A & Unused - Few or no signs of age and storage may be seen.

Cleaning:
Dry Clean Only!
Air outside on a dry day a couple times a year.

Quality Rank (Outer Fabric):
S = Near Mint Condition, No signs of aging, staining, or wear.
A = A very small amount of staining or aging can be seen, such as one or two age spots.
B = Signs of use and a few small stains or many tiny stains (such as dots from aging).
C = Stains are easy to spot, large, and/or numerous, and may contrast heavily with the fabric.
D = Holes or very large stains. Significant signs of aging and use may also be seen.
E = Salvage only.