To add or enter translations for other contexts on the translation screen, select the 'Show System Tabs' button. This will display a tab for each additional context in Workbench (Captions being the first tab).
The data on these additional contexts are not specific to the 'Target Form' (Screen being translated) and rather are a list of all of the text currently available within that context for translation. The only exceptions to this are the Captions context (The first tab shown here as the 'Job Management' tab) and the 'Messages' tab, which can be filtered to show only those messages which relate to the 'Target Form' (Current Screen or Workbench Object).
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