title: What does this Account storage map definition mean in the pallet_balances?
body: <pre class="lang-rust prettyprint-override"><code>#[pallet::storage]
      pub type Account&lt;T: Config&lt;I&gt;, I: 'static = ()&gt; = StorageMap&lt;
          _,
          Blake2_128Concat,
          T::AccountId,
          AccountData&lt;T::Balance&gt;,
          ValueQuery,
          GetDefault,
          ConstU32&lt;300_000&gt;,
      &gt;;
  </code></pre>
  <p>What does <code>ConstU32&lt;300_000&gt;</code> mean here? This definition is from the balances pallet.</p>
  <p>Also when are we supposed to declare storage maps as <code>pub</code> rather than <code>pub(super)</code>?</p>