It is commonly agreed upon to use spaces and forward slashes in code to streamline and reduce its size through minification. However, when performing a specific action, it is more likely that the expanded form will be utilized.
Whether the expanded form is used for clarity or brevity often comes down to personal preference.
For some individuals, using shorter notations may simply be a matter of habit. Ultimately, the choice between shorthand and full notation comes down to your coding style and preferences.
Typically, slashes indicate different functions within code, although this may not be as common in CSS. Spaces are frequently used to separate arguments, such as in the case of the transition property:
transition: duration (e.g. .55s) || transition_type (e.g. ease) || action_type (e.g. all);
In this example, spaces (indicated by ||) differentiate between elements like duration, transition type, and action type for the same CSS property.
This practice can also lead to slight reductions in file size, albeit not significantly - just an added bonus :)