You can define as many geometries as you want. Angle has no such limitation.
In order to assist with data entry Angle – while entering/editing/reviewing the data – always displays a schematic drawing representing the data which is being currently entered. When entering the dimensions, a red dimension line is showed on the illustration indicating the required dimension. So, simply look at the schematics while entering the data.
Although you do not actually use a holder, you can still define a “virtual” one, in order to account for the distance between the source and the detector. Just define a holder with a cap thickness equal to zero and a radius bigger than both the detector and source. The only thing you will have to precisely define is the holder height, which will then represent the actual distance between the source (or its container, if present) and the detector.
The additional absorbing layer is any material encountered by gamma-rays on their way from source to detector, e.g. a plastic bag over the detector’s end-cap (commonly used to protect from contamination). For each layer you can define its top and side thickness and material.
Yes. The distance between the detector and source is automatically increased by the sum of the top thicknesses of all defined absorbing layers. In other words, the holder and/or container (if any) are put on top of those layers.
No. Simply choose the geometry you would like to use, click “Edit”, change whatever you want, click on the “Save as” button and give a name to the new geometry. A good tip for the counting optimization or error propagation studies!