The problem is that the these two languages are read from right to left and can have bi-directional data such as mixed Hebrew and English on the same line. It is based on this information that a new character set is being defined which will indicate that the bi- directional message is either encoded in implicit mode or explicit mode. The default is visual mode which requires no special character set other than the standard ones previously defined by ISO-8859. Implicit directionality is a presentation method in which the direction is determined by an algorithm according to the type of characters and their position relative to the adjacent characters and according to their primary direction. The complete algorithm is quite complex and sites wishing to implement it should refer to the ECMA Technical Report for further details.
All text is viewed in the same direction which is the primary display direction.
The displaying application is not aware of the contents direction and displays the text as if it were a uni- directional text. The composing application needs to prepare the text in such a way that it will be displayed correctly. No control characters or algorithms are used to determine how the data is to be displayed. This is the simplest of all methods and the default method for use with MIME encoded texts. 3rd edition, December 15, 1992.