English to German Bus/Financial Insurance

Aus der Beschreibung eines betrieblichen Altersversorgungsplans:

The 401(k) Plan has a “matching” contribution feature under which XX may contribute a percentage of the amount deferred by each participant, excluding the “catch-up contribution.” Such percentage, if any, is determined by the subsidiary’s management at its discretion.
The Plan also has a discretionary “profit sharing” feature, under which the subsidiary may contribute, at its discretion, an additional amount allocable to the accounts of active participants meeting the aforementioned eligibility requirements.
XX's contributions to the 401(k) Plan participant **vest ratably** during the first five years of a participant’s employment and 100% thereafter.

Wie ist hier das "vest ratably" zu verstehen? Werden die Beiträge "nach und nach"unverfallbar? oder anteilig? Oder liege ich mit "unverfallbar" sowieso falsch?