Are you saying this is how you have your Bubble database set up? You have a Thing with 400 fields on it?
As @tylerboodman implies, this is not how you do it.
Bubble basically stores JavaScript objects and it’s indexed. Those objects (which Bubble calls “Things”) have a Type (you create a name for it) and then those objects have fields which themselves have a data type and the data type can even be some other Thing. Things are referenced by their Unique ID - it’s basically the “primary index” for that unique object. When a Thing has a field of Some Other Thing type, and you store a value there, what you are storing is a reference to that Some Other Thing (its unique ID). And a field might even be a LIST (an array) and so have multiple Things in it, again internally they are simply referenced by their Unique IDs.
So, in your app, you have 300 Categories. Each of these presumably has some number of Subcategories, yes? So here’s what a Category looks like (at a minimum):
And then Subcategories apparently have some number of Specifications on them, so here’s what a Subcategory looks like (at a minimum):
And then you have hundreds of Specifications. You need to make these, just like the individual Category and Subcategory objects you’ve already made. They would look something like this (you don’t say what’s in a “Specification”):
One you create your individual Specifications, you take your individual Subcategory things and populate each Subcategory’s Specification List with the relevant Specifications, but putting each relevant Specification into the Specification List field.
(And of course, obviously we don’t do that in the data tab, we make a little admin page where we can make a better interface for doing so, which seems to be what you’re working on.)
Similarly, for each Category, you will populate its Subcategory List with all of the relevant Subcategory objects.
And now, when a user selects a category, they can see relevant subcategories and when they select one of those, you now know all of the Specifications relevant to that Subcategory.
That’s it. Simple.