Drawing

sabir,
You have asked a very basic but also a very logical question.
"How can we differentiate a Piping Layout, Plant layout, Plot Plan and General Arrangement drawing (GA)"

I will try to give you some answers and make them understandable since I cannot be in front of you with samples.

First, some of these terms are "work activities" (or "a work process") and some are the "deliverables" (or the "product") of a work activity.
Lets start by putting them in the order we would normally see accomplish them.

Plant layout: This is first a "work activity." It is the process of determining what Area you have for a Plant, the equipment or units you need to fit into the Area and the exact relationship these items must have relative to each other. In the past It has been done with manual sketches, paper cut-outs and small scale block models. Today it can be done in "Model Space" by moving around circles, rectangles and squares, as long as you keep it simple. Remember, this is an activity, it is not a deliverable. You can call it the Plant Layout, but it is still not the deliverable.

Plot Plan: This is a "deliverable." After the overall plant layout or unit layout has reached a point where the actual equipment number, sizes and shapes are known the "Model Space" version of the plant layout can be up-dated and formalized. It would then moved to "Paper Space", a Title Block added and the formal Plot Plan produced. This document now goes to various people for review and approval including the Client. Because of the timing, the scale and the purpose of this Plot Plan document it would not and should not have any coordinates or dimensions on it to locate equipment or facilities.

General Arrangement drawing (GA): (Also known as the Equipment "Location Control Plan" (LCP): This document is a limited internal deliverable tool that would not and should not be issued to the job site. This is the more detailed step to fixing the location of each piece of equipment in to it's final location. This final location will be based on the purpose, function size of that piece of equipment along with the piping layout, operation space and the maintenance space. In addition to this there is the space requirements for any related electrical equipment and instrumentation requirements. The final location of each piece of equipment is also affected by the foundation design, and the foundation design is impacted by the type and stability of the soil at the site. The final location of a piece of equipment should only be conveyed to the field by the document (maybe called Foundation Location Plan) from the Structural department.

Piping Layout: This is first an activity (or a process) and then a product. A piping designer does piping layout. This is the process of defining the piece of equipment, the maintenance spaces and the related electrical needs. Then routes the piping to accomplish the process function of the piece of equipment then add the space requirements for operation and you have a piping layout. The final location of any piece of equipment is impacted by the piping layout. This final location is added to the "GA" (or LCP) which is then routed to the structural group. As you can see there is a normal understood cycle and recycle between the piping group with the piping layouts, the "GA's" and the structural group and their "Foundation Location Plan".

I hope this affords you some insight to the meaning of these terms. Remember these are only my thoughts, I hope others will add their thoughts also.