Best documentation reference for KoBoCollect with XLSForms

I would like to better understand how I can find the REALLY relevant background information for designing and working with KoBoCollect (and Toolbox), based on XLSForms (in Excel.

What is the best documentation reference for KoBO Collect implementation??
There are several references cited, like:

For example:

  • I find things differently specified, and some not (fully) implemented like XLSForms spec in KoBo Toolbox, like “range” or “hidden”. (See also:

For example, if you want to find out, which option can be used in “default” (XLSForms column) or which ones can be used in choice_filter (XLSForms column):

  • Expressions, like if condition?, calculation?, function (e.g. today() )?
  • Variable reference (e.g. ${today})?
  • Simple static value?
    Or if you want to find out, if in choices also variable references can be used.

What is exactly the base (and main documentation reference) for KoBoCollect (via XLSForms spec in Excel)?

  • Is this XForm?
  • XForm as supported by Javarosa?
  • XLSForms?
  • A specific KoBo version … (of what?)?
  • A specific version of ODK (Collect?)?

I am sorry, it is a bit difficult to get it clear:

  • “XLSForm is a form standard created to help simplify the authoring of forms in Excel. The XLSForm is then converted to an XForm, a popular open form standard. … XLSForms are compatible with the subset of XForm functionality supported by Javarosa Project”
  • “ODK Collect renders forms that are compliant with the ODK XForms standard, a subset of the XForms 1.1 standard with some extensions. The form parsing is done by the JavaRosa library which Collect includes as a dependency.”
  • KoBoCollect (github): “Clone of ODK Collect for use in the KoBoToolbox”.
  • “KoBoCollect is based on the OpenDataKit” (google play store)|
    Maybe someone could draw a picture how the KoBoCollect things are related?

Is there some similar KoBo Collect reference like this?

Thanks in advance!