Select the rectangle / custom / ellipse etc. Go to the control bar of Photoshop and in the middle, the shape / path tool options. Go to the gear icon in the control bar and you will see all the available entries
Rectangles, ellipses, triangles etc all have the same core features such as constrained, apply proportional, fixed etc. They all have their uses as you apply the vector designs. Some such as polygon, have a few additional features for stars etc
Select unconstrained, defined proportions, defined size, fixed size etc
Select rectangle / custom etc. Select defined size (set size as required). Go to the document. Click and repeat click to add the fill over the entire document (great for a nozzle like effect for shapes). The defined proportions applies the artwork as defined the proportions set by the original design before being saved as a custom shape. Great for keeping the presets in proportion to one another. So if you have a preset with a size of 10px by 10px, you can then set it as 100px and 100px or 200px and 200px or 2px and 2px etc and they will all be in that proportion. If the vector artwork was created with a size of 100px (horizontal) and 200px Vertical) then any created presets using 'defined proportions' will be 10px / 20px or 30px / 60px etc
This just uses the preset as was originally set which may be of little use if the created size was quite small. Many of my vector sets were created with documents sized about 300 x 300px which means that the defined size is around that value but you may find others have defined them at 2000 x 2000 etc. Still, it is useful for adding the artworks with a set size that you may have defined for a particular project such as perhaps having a building size or architectural item or an engineering shape set to 1000px and 500px and then whenever you use it in your projects, the defined size command adds the artwork exactly to that size.
The fixed size is useful for a nozzles like / brush apply of the shapes. You can set the fixed size to 100px by 100px etc and all the shapes will then be applied (whether in proportion or not) the same size over and over by a click of the document. You can also drag the shape into position. It is a pity there is no 'pressure sensitive setting' so the fixed size can be increased or decreased based on the pressure to the maximum set 'fixed size'
The default option and possibly the easiest to use to apply the design as you want and in any position or proportion. They work with all such as rectangle, polygon etc. Click unconstrained. Apply the preset and stretch and distort left and right and top to bottom.
Probably the oddest one to work with as it just creates the artwork from the center (click) outwards. However it does have a use in that it makes it very easy to line things up especially when you combine this with grids and guides - so view menu and add a guide and then use the from center to position the design each and every time with that as the center, no need for alignment
The sets can be deleted by accident, if that has happened to you, you can restore the Adobe ones very quickly by going to the window menu and shapes and then going to the right side menu of the displayed panel and then selecting the append command to return the current designs and so select the legacy command to return all the old legacy presets to the panel.