Actually I found the answer the next day within the same manual ( not explicit but easy to understand )
As you can see there is a vertical black line denoting a revision mark . If there are any revisions , Airbus issues a “summary of highlights” table at the begging of each chapter containing a list of things that have been revised ( deletions / additions , rephrasing etc) . So for the documentary unit you see in the attached picture there where 2 revisions made and for each( numbered 1 and 2 in this example ) , a brief explanation is given in the summary of highlights table .
If you happened to have the very latest FCOM ( I downloaded it 4 weeks ago ) you may notice that in the PRO-NOR-SUP section there are quite many (minor) changes , e.g you may see a revision mark with numbers from 1 to 8 . It means that within the vertical black bar denoting a revision mark ,8 changes were made ( roughly adjacent to the position of the digit ) and you can read those in the corresponding “summary of highlights” table .