I am reviewing a model where partial moment releases have been used and both MPY and MPZ have been specified as 0.99. What does that mean ?
0.99 value in partial moment release would mean that 99% of the moment carrying capacity at that member end has been released. In other words it means that whatever moment the beam end connection would have carried had it been a full moment connection, with the applied 0.99 factor, it would now be able to carry only 1% of that moment.
For example, if the full moment type connection at the beam end would have carried 90 Kip-ft, with a 0.99 moment release, it would now have the ability to carry 0.01x 90 Kip-ft = 0.9 Kip-ft. In general partial moment releases are used when instabilities are caused due to application of full moment releases. In such cases the model is stabilized by specifying partial moment releases as opposed to full moment releases, the logic being that even if you have a shear connection, realistically it has some ability to carry moments. That limited ability of a shear connection to carry moments is modeled by specifying a partial moment release factor of around 0.97 to 0.99 ( meaning 97 to 99% of the moment carrying capacity at the joint is released). You should avoid using values less than 0.95.