The USAF system has four tiers, and the lowest-altitude layer is the suggestively named tier N/A, which was carved out of tier 1 in prospective recognition of the unique qualities of mini-UAVs.
美国空军系统分为四层，高度最低的一层是建议名称为N/A层，小型无人机的独特性质正是该层区别于第一层的主要标志。
The USAF system has four tiers, and the lowest-altitude layer is the suggestively named tier N/A, which was carved out of tier 1 in prospective recognition of the unique qualities of mini-UAVs.
美国空军系统分为四层，高度最低的一层是建议名称为N/A层，小型无人机的独特性质正是该层区别于第一层的主要标志。
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