I am a Masters student and quite new to ArcGIS. I am working on a map and am a little confused about when to use transformation of coordinate systems/projections. I am using ArcGIS v10.2 and have a map with two coordinate systems: GCS GDA 94 and UTM GDA 94 as well as a basemap. I understand that ArcMap projects these on the fly, but I am wondering if I need to transform the GCS GDA 94 data to UTM GDA 94 to edit the data. By editing the data, I mean adding polygons as layers and using the AddIn Sampling design tool to add random points to these. I sometimes get warning messages from ArcGIS about this. I have read quite a few posts on Esri's site, but am still a little unclear. I need to export the final GPS points as UTM GDA 94 to transfer to my GPS.
If you go to the data frame properties and then the coordinates tab. Choose the coordinate system you want (lets say UTM) then there is the transform button at the bottom, click on that. You will see a FROM to INTO and then the Using, it's that Using drop down that offers up the actual transformation. I've never used what you are using so it may only have just one choice? A Google search is throwing up various threads and pages about it.