Any company or client will have a set of rules , in terms of there branding , packaging an marketing , that any designer would have to adhere to. A style manual/guide is standard for a new designer to be handed when undertaking any project.
The guidelines can consist of several features, from signage, the typeface allowed to use, the size an weight of said typeface, the colour of the logo, most will have a specific composite of colours. Space between the images & text. Minimum and maximum sizes of font. Hierarchy of how fonts & images are laid out.
The reason for these stringent set of rules, is to have complete fluidity throughout your brand. The advert can look different have a different take, but still your logo remains the same, therefore aiding the brand identity, keeping a uniform sense.
The logo being a big part of the brands assests. There will be guides as to how big an small the logo can be stretched to. Where it can be placed. The size of the text next to the logo, whether there any allowed next to it in the first place., general spacing around the logo.
Some famous brand an there brand guidelines:
Ikea – Ikea Brand Manual
Macdonalds – Macdonalds Brand Guideline
Just worth being aware that when working fora client, you won’t have as much free rain , as first thought. Having to follow guidelines could help you from not getting into trouble, if you work for a client an don’t portray there message correctly. They won’t pay yo for the effort if you haven’t got the job right.