The very first header of a document can be of any level, although we highly recommend your document to start with a \H[1], and to contain exactly just one \H[1], as this has implications such as:
After the initial header however, you must not skip a header level, e.g. the following would give an error because it skips level 3:
= my 1

== my 1

==== my 4


  1. Header
  2. Macro
  3. OurBigBook Markup
  4. OurBigBook Project