Moodle allows grouping of activities in a construct called "Section". Depending on the "Course format" selected in the settings of a module, sections appear either as Topics or Weeks. This is simply cosmetic, as Moodle internally will always think of them as sections.
Sections are usually numbered sequentially depending on the time they were created and not their placement. So if you have 5 sections and you add a sixth one and move it at the top of your module, it will not become "section 1". It will still remain "section 6" as it was the 6th section to be created. There are cases where school or institution specific implementations have been created that don't follow this rule. However, the guidance below will work in every case, and should work for your Moodle version too.
Moodle backup files are created in a rigid and serialised way that does not allow redirection of activities to different sections during restore. This practically means that if an activity is added in Section 2 in the backup it will always be restored in section 2 in the target module. It is important then to place activities in the correct place in the backup, if we want them to appear where intended in the target module. This is why LD asks you to choose which section you want to add the content to.
Finding the section number is usually "Course format" specific. Choose the "Course format" of your module from the list below, to see how to find where Moodle might show you the number of the section:
In topics format it is very easy to find the section number. Simply hover over the topic and check the URL shown in the status bar.
In the example above you can see that Topic 3 is section 3. This means that if you want the content generated by LD to end up in Topic 3, then in the popup you need to enter 3 as the section number.
In Weekly format, sections are automatically named depending on the start date of the course, as set in module settings. In the example below
we see that the section for week commencing 9th November is section 8.
This means that if you want the content generated by LD to end up in week commencing 9th November, then in the popup you need to enter 8 as the section number.
In Collapsed format, it is possible that hovering over the name of the section will not give you the URL in the status bar. If this is the case,
you can use the single section button that exists on the right-hand side of the section title, as shown below.
In the example above we see that Week 5 is in fact section 6. This means that if you want the content generated by LD to end up in Week 5, then in the popup you need to enter 6 as the section number.