Scenario
An external GPS Exchange Format ('GPX') file has been previously and successfully parsed. Parsing has split the file into potentially three subsets: waypoints, routes and tracks.
You have chosen to import one of these three subsets and are being asked to supply the specifics of their importation.
NB: Individual waypoints, routes and tracks can be imported. Go back to the file analysis (previous) screen and read its help book.

Screen layout field descriptions

Notes:
These are self-evident. Return to the main screen and select Backup/Restore from the hardware menu to preserve a copy of your database prior to importation.

File name:
Information only. Last moment confirmation that you are importing the desired file. The location prefix symbols are:
♠=SD card root directory
♦=SD card/Download directory
♥=sample file pre-supplied in 'assets'.
[To see the samples, use the 'spanner' icon (Settings) on the main screen action bar and check Expose sample .gpx files.]

Subset type:
Information only. Last moment confirmation that you are importing the desired entities (Waypoints, Routes or Tracks).

Subset quantity:
Information only. The number of entities (Waypoints, Routes or Tracks) about to be imported.

id0: and id1
[Also see Manage user-defined key-value pairs (the 'face' icon on the main screen).]
Two of these these keys, id0 and id1 are used extensively to filter your data and provide you with lists containing only items relevant to the moment...
The activity will always propose an id0 value same as the name of the GPX file being imported. If, for example, you are about to import a whole lot of waypoints that mark features on Joe's farm you might like to overtype it with something like Joe's Farm. This value will be permanently remembered as a new key-value  pair.
[The id1 value is used as a further method of filtering, allowing you to break Joe's farm into paddocks, quadrants or some such.]
Or, assuming you have previously set up your own set of id0/id1 key-values, select the desired value from the drop-down spinner list to the right of the id0/id1 screen fields. Then Long Press the spinner to copy the value into the field.
During importation, the selected value(s) will then be stored as part of the imported entities.

[CheckBox] Force use of above id0/id1 values
[If you are importing a new GPX file this CheckBox has no effect.]
It deals with the situation where GPX data previously exported from this app is now being re-imported. Action is as follows:
If CHECKED, the above id0/id1 values will always be used for all imported entities.
If NOT checked, the above id0/id1 values will be used only when empty or non-existant <id0> and <id1> XML tags are found in the importing file. If non-null tags are found, then their values will be used instead.

[CheckBox] Default date
When a <time> XML tag is found in the importing file it's value will be imported into the new entity.
This CheckBox determines action taken when empty (or non-existant) <time> tags are encountered. Select None to do nothing or Today to use the current date and time.

[CheckBox] Importation mode
When a <name> XML tag is found in the importing file it's value is used to name the newly imported entity.
Select Replace!!! to perform a database query before adding the entity. If an entity with the same name, id0 and id1 as the importing entity is found, it's data will be destructively replaced with the new data.
Select Merge to bypass the above query and allow for possible duplication. (i.e multiple different entities but with the same name.)

Symbol alias binder
Symbol values can be transformed via alias specification during both import and export. (A more detailed discussion of this is found in the 'Export GPX' help.)

[See Manage user-defined key-value pairs (the 'face' icon on the main screen).]
Set up any number of aliases, binding them together using a common description as a binder. For example set up an alias as follows:
       S1 ('String 1') value = boat
       S2 ('String 2') value = ship
       Description = #symBinder2
Return to this program. Alongside this field the drop-down list ('spinner') will contain a list of all alias binders. Short press the drop-down list to select #symBinder2 then long press it to paste in the value.
Now, during import, all symbol values of boat will be transformed to ship.

Finally...
Press Import to begin updating, or Cancel