Hi @AndrewGardini and welcome! 
User Specific
User specific columns are often used to store notes, comments, preferences. Also for adding an element to favorite contents (cf. User-specific columns | Glide Docs).
For example, you could let the user select the icons of the Options table by making the column where they are available User specific. This way, if one don’t likes the red/green colors or shapes, (s)he would be able to make it more personnal.
Template
Just to be sure: your objective is to have multiple users on your app, each with their own data, right? And following from that, each user will have their own email address to log in?
In this scenario, the only way to ensure your app is secure (i.e. users only have access to their own data, no matter what) is by using the Row Owner feature. Here’s the documentation: Row Owners | Glide Docs
I checked the template you mentioned (hoping it’s the right one… Personal Finance | No Code App | Glide), and it was built for single-user use only.
To adapt it for multiple users, here’s what you probably need to do:
- Add a column in the
Transactions table to store a unique user identifier (e.g. their email address), and mark this column as a Row Owner.
- When a user adds a new transaction, their email address will be automatically written into this column. This ensures that only they will have access to that row.
Row Owner or User Specific?
Tables like Categorie are not concerned by the previous point because they aggregate values from the Transactions table.
However, you could implement Row Owner in your Users table or in any other table that contains user-specific data.
To give you one last example (hoping it helps): even though Row Owner is not relevant for the Categorie table, a Notes column with the User Specific setting enabled could make sense.
Each user could add a personal note to better understand the transactions listed in a given category.
For example, you have two similar categories: Others and Unknown. One user might use just one of them as a “junk” category and would appreciate being able to leave a note as a reminder of that choice — while another user might do something completely different.
In this case, everyone sees the same list of categories, but thanks to the User Specific feature, each person can add their own comment 
Glad to clarify further if needed!
