In this option, the contents of the central unit cell plus others within specified limits are accepted. The central or "zero" unit cell contains atoms with fractional coordinates 0.00000 to 0.99999. The lower limit to unit-cell translations is normally zero or negative and the upper limit is normally zero or positive. Disabling the translation entirely for an axis is equivalent to setting the limits at zero. The translations belong to the Bravais lattice, not the primitive lattice.
As an example of application to a crystal, putting the lower limit at -1 and the upper limit at 0 for all axes results in a box with the shape of the Bravais unit cell, centered on the origin and containing eight unit cells. Note that the "zero" unit cell, containing atoms with fractional coordinates 0.0 to 0.99999, extends in the positive directions of the axes.
This is the primary option for one-dimensional polymers. It is also the simplest option for two-dimensional polymers, but the shape of the resulting sheet is limited to the shape of the unit cell, always a parallelepiped. You can use the Enter Forms option to give a different shape to sheets.