An open structure is a term used to convey to STAAD that wind blows through the building and the only obstruction encountered by the wind are the widths of the individual members (beams, columns and braces) of the structure. Lattice structures like electrical transmission towers are considered open structures. In such cases, STAAD calculates the load due to wind by multiplying the member width (or some component of it depending on the BETA angle) by the wind pressure to come up with a uniform distributed load that acts on the length of each member.
For the other type of wind (the non-open category), the load due to wind is obtained by multiplying the area enclosed within the panels formed by the members and multiplying that area by the wind pressure to come up with a total load which is then applied as a number of point loads at the various nodes of the panel.
Besides section 5.31.3 of the Technical Reference Manual, you can find a detailed description and examples of the Wind load feature in section 2.1 of the book written by Kris Sathia.
https://store.bentley.com/en/products/books/WAD00908A10001--Principles-of-Structural-Analysis
If you are based in India, you can contact the Bentley office near you for the Indian edition of the book.
The MP method of releasing the moments at the start or end of a member involves specifying a fraction that represents an estimate of the amount of the release. For example, if your estimate says that the connection type at a member end will transfer only about 5% of the moments that would be transferred by a fully moment-resistant connection, specify MP as 0.95 (meaning, a 95% release). In STAAD, you can specify individual values for MPX, MPY and MPZ. A shear connection (connection that is assumed to have no moment transfer ability) can be specified by setting MPY and MPZ to 0.99. MPX is the variable representing a torsion release, but from a practical standpoint, it is not easy to see how torsion can be released.