inPlace=True means that initial_transform is modified by the registration, if it is False then the transform isn’t modified. Please see the documentation for details.
The resulting transform maps points from the fixed image to the moving image. Please read the Registration Overview and the Fundamental Concepts pages to gain better understanding of how transforms are applied.
I saw in the link you presented the following text:
Features of Interest
Transforms and image spaces
While the goal of registration, as defined above, refers to a single transformation and two images, the ITKv4 registration and the SimpleITK ImageRegistrationMethod provide additional flexibility in registration configuration.
From a coordinate system standpoint ITKv4 introduced the virtual image domain , making registration a symmetric process so that both images are treated similarly. As a consequence the ImageRegistrationMethod has methods for setting three transformations :
SetInitialTransform ToTo - composed with the moving initial transform, maps points from the virtual image domain to the moving image domain, modified during optimization.
SetFixedInitialTransform TfTf - maps points from the virtual image domain to the fixed image domain, never modified.
SetMovingInitialTransform TmTm- maps points from the virtual image domain to the moving image domain, never modified.
The transformation that maps points from the fixed to moving image domains is thus: