Sequential Workflow tasks are always contained in a workflow block. The basic workflow block is the sequential workflow itself. Blocks can be nested, so one block can contain not only tasks, but also other blocks. Blocks can be used for just grouping the tasks together (Transaction Block) or for repeating operations (For-Each, Loop).
All the context stores contained in the parent block are also visible to the current block. I.e. context stores defined in the workflow will be visible to all the blocks defined in this workflow. Additionally, in each block you can define its internal context stores. The lifetime of the context store depends on the lifetime of the block. After the block is finished, the data of the context is removed from memory.