这里that作主语引导定语从句，先行词为digital materials. 因为定语从句比较长，如果放在前面和主句连在一起容易产生歧义，也不合乎尾重原则，因此此时需要把定语从句后置，语法上称为“分割式定语从句”，这种现象在英语里很常见。例如：

Tidal sites have already been identified that will produce one sixth or more of the UK's power——and at prices competitive with modem gas turbines and undercutting those of the already ailing nuclear industry.