Change:
type: 'string', enum: ['always', 'never', 'whenTriggerCharacter', 'whenQuickSuggestion'], enumDescriptions: [ <del> nls.localize('suggest.insertMode.always', "Always activate the suggest widget when triggering IntelliSense automatically."), nls.localize('suggest.insertMode.never', "Never activate the suggest widget when triggering IntelliSense automatically."), nls.localize('suggest.insertMode.whenTriggerCharacter', "Activate the suggest widget only when triggering IntelliSense from a trigger character."), nls.localize('suggest.insertMode.whenQuickSuggestion', "Activate the suggest widget only when triggering IntelliSense as you type."), </del> <ins> nls.localize('suggest.insertMode.always', "Always select a suggestion when automatically triggering IntelliSense."), nls.localize('suggest.insertMode.never', "Never select a suggestion when automatically triggering IntelliSense."), nls.localize('suggest.insertMode.whenTriggerCharacter', "Select a suggestion only when triggering IntelliSense from a trigger character."), nls.localize('suggest.insertMode.whenQuickSuggestion', "Select a suggestion only when triggering IntelliSense as you type."), </ins> ], default: defaults.selectionMode, <del> markdownDescription: nls.localize('suggest.selectionMode', "Controls whether the suggest widget becomes active when triggered via quick suggest or trigger characters. Note that the widget is always active when explicitly invoked, e.g via `Ctrl+Space`.") </del> <ins> markdownDescription: nls.localize('suggest.selectionMode', "Controls whether a suggestion is selected when the widget shows. Note that this only applies to automatically triggered suggestions (`#editor.quickSuggestions#` and `#editor.suggestOnTriggerCharacters#`) and that a suggestion is always selected when explicitly invoked, e.g via `Ctrl+Space`.") </ins> }, 'editor.suggest.snippetsPreventQuickSuggestions': { type: 'boolean',

Discussion:
Testing Here is the description of this setting Controls whether the suggest widget becomes active when triggered via quick suggest or trigger characters. Note that the widget is always active when explicitly invoked, e.g via Ctrl+Space. It was not clear to me what active means. After trying around I figured out that its about the default selection of the first suggestion in the widget. Setting itself calls rather than . Hence I would recommend to explain in the description that this setting is about controlling the selection of the suggestion. Like Controls the selection of the options in the suggest widget when triggered via quick suggest or trigger characters. Note that the first suggestion is always selected when explicitly invoked, e.g via Ctrl+Space.
It's not always the first suggestion - there is yet another setting to define what element is default-"selected". The problem is that technically this isn't selection (because selecting inserts the item) and also not focus (because focus remains in the editor). I used selection mode because that's how VS calls it but I understand the issue and I open for a better setting name and better description. Maybe we should just talk about focus?
I think keeping selection isn't bad but maybe the working should say "Controls whether a suggestion is selected when triggered via quick suggest or trigger characters..."