看不大明白，求解释

justlooks

手册中innodb compact存储描述
For each non-NULL variable-length field, the record header contains the length of the column in one or two bytes. Two bytes will only be needed if part of the column is stored externally in overflow pages or the maximum length exceeds 255 bytes and the actual length exceeds 127 bytes. For an externally stored column, the two-byte length indicates the length of the internally stored part plus the 20-byte pointer to the externally stored part. The internal part is 768 bytes, so the length is 768+20. The 20-byte pointer stores the true length of the column.

对于非NULL变长列，记录头部用1到2个字节记录列的长度 ，当列的一部分存储在外部或者最大长度超过255，实际长度超过127bytes，对于部分外部存储的列，2个字节长度表示在内部部分的长度加上20字节指向外部存储部分，这个内部存储和外部存储应用与什么情况？？