Extract distinct values from XML based on attribute value

You can use XMLTable with an Xpath that targets the Kod=N nodes, then walks back up the tree to get the doc_num; this version assumes your original value is a string:

select distinct xml.doc_num, xml.x
from your_table t
cross apply xmltable (
  '/main/item/Dok[@Kod="N"]'
  passing xmltype(t.your_xml)
  columns doc_num number path './../../@doc_num',
    x number path '@X'
) xml

DOC_NUM X
------- -
     99 1
     99 4
     99 5

db<>fiddle

If your original value is already XMLType then walking back up the tree doesn't work as expected - not sure if that's a bug but seems to happen in 11gR2 and 18c at least. You can work around that by converting to a string and back:

select distinct xml.doc_num, xml.x
from your_table t
cross apply xmltable (
  '/main/item/Dok[@Kod="N"]'
  passing xmltype(t.your_xml.getclobval())
  columns doc_num number path './../../@doc_num',
    x number path '@X'
) xml

DOC_NUM X
------- -
     99 1
     99 4
     99 5

db<>fiddle

I've just noticed you tagged this for Oracle 11g... so cross apply isn't available, and walking back up the tree doesn't work either; neither does filtering on the child attribute name. So you can do this instead, with two levels of XMLTable:

select distinct xml1.doc_num, xml2.x
from your_table t
cross join xmltable (
  '/main'
  passing xmltype(t.your_xml)
  columns doc_num number path '@doc_num',
    doks xmltype path 'item/Dok'
) xml1
cross join xmltable (
  '/Dok'
  passing xml1.doks
  columns kod varchar2(1) path '@Kod',
    x number path '@X'
) xml2
where xml2.kod = 'N'

DOC_NUM X
------- -
     99 1
     99 4
     99 5

which works in 11g with a string or with an XMLType source.