The Tin Block is bulk storage for your Tin Ingots. It can also be used as an attractive and unusual metallic looking decorative block.
It can be crafted out of 9 Tin Ingots from Galacticraft. If you have another mod installed with a different variety of tin, normally you can mix and match the ingots to craft a Tin Block.
(In GC3 only) a Galacticraft Tin Block will be made from mixed tin ingots if you use at least 1 Galacticraft Tin Ingot in the recipe - but if you use zero Galacticraft ingots then a Galacticraft block will not be made. This recipe requirement for minimum 1 Galacticraft ingot allows you to control whether the Tin Block from Galacticraft is made or the tin block from the other mod.