Bar diameter and where it goes
On a typical Zanzibar residential or commercial frame, smaller diameters do the distribution and link work while larger diameters carry the main structural forces in columns, beams and foundations. The drawings assign them; substituting a diameter because it is what the yard had that week is a structural decision, not a procurement one.
If a diameter on your schedule is difficult to source at the time you need it, we will say so at quotation stage so the engineer can be consulted, rather than swapping it silently.

