The Blocks tab allows selecting the blocks on which the order line will be allowed to be scheduled. Blocks can be added by pressing the [Add] button and ticking-off the blocks to be used. The list of blocks will contain only those blocks which are valid (within the order's validity period).
NB: Depending on whether the "Add blocks for single-types rather than type-groups" is enabled or not, it will be displayed here either only those blocks with segments matching the event type specified in the "Parameters" tab, or all those with segments matching the group to which the specified event type.