In some configuration tools, you can select multiple options in one go, and then the engine tries to install them into the box. This is called a post-processor. PSCFG operates quite differently. When you click on the Memory tab, for example, it first checks all the partnumbers in the database, and selects only those that are classed as Memory parts, then filters a second time to see which ones are supported on this specific model, and finally a third time to see if they will fit at this precise moment. (All of this happens in a tiny fraction of a second.) If it is a valid option after the second filtering, the partnumber is displayed. If there is not enough room for it, or something like a connector is missing, the partnumber is displayed, but with an X in the first column. (See the light blue arrow #1 on the left, below.) This means, in essence, "This is a valid option for this model, but I have already checked to see if you could add this partnumber, and it won't fit at this precise moment." You can then use mouse button 2 to click on Conflict (see arrow #3 below) which causes this small window shown below to pop up. Inside it will list the partnumber it cannot add, the Description, and what is required to resolve the problem. (You can even track what system resources you have, by using the Resource Monitor window at the bottom (arrow #2) and watch the resource decrement every time you add an option that consumes it.)
Sometimes it is easy to resolve the conflict, by adding a second processor to a Server whose memory sockets are half-full. In other cases it is not possible, such as adding another memory socket, or a special type of internal slot or bay. You may have to accept that you must either pick a bigger memory module or hard drive, or make do with what you have so far. Displaying supported options is a dynamic function - options that are supported only when a pre-requisite item is added, will not become visible until the pre-requisite has been selected. One example of this would be the ThinkPad UltraBase. Options not supported on the base system unit will not be visible until you add the UltraBase.
But what if you understand you cannot add the item in question, but you want to make it part of the configuration anyway ? Perhaps the customer needs to compare two possible choices (with everyone knowing only one of them can fit at one time) and then decide on which of the two he/she will order, based on the technology, price & supply. PSCFG includes a dummy part that will allow you to 'break the rules' and over-configure the system. A warning is included in the resulting configuration, that anything added after that dummy part, may not actually fit, but is included for comparison purposes.