The "Lot/Phase" is an attribute you can use to split your model in parts: "Phase 1”, "Phase 2"... it's useful if you have a big job and you would like to detail, fabricate or send the job in site by parts not all in the same time.
The "Lot/Phase" it’s also good for numbering. If the check box in from the field "Lot/Phase" is checked and if you have 2 identical pieces but one is in "Phase 1" and the second is in "Phase 2" they will get 2 different part marks.
Also is good for the pieces marks. After the numbering is done you can select all the element from "Phase 1" and you can start creating the drawings with 100 like this all the elements from "Phase 1" will have the pieces mark starting with 1XX, after that you can select the elements from "Phase 2" and you start creating the drawings with 200 and you can continue like this depending how many phases you have in your job. Using this method you know for sure that the beam B156 is from phase 1 and the beam B239 is from phase 2
Also you can use this to do list by phases, like this you can know the necessary material by phases and by all job.
This spilt can done by parts of your job from left to right, from right to left but also you can go by floors (levels).