Regardless of type, all body modifications might take place before, during, or after an important ceremony or event, such as a wedding, childbirth, or age milestone. They therefore signify a rite of passage having taken place, and the absence of the expected modification indicates it hasn’t happened. These can both come with judgments, positive or negative, about the value (italics) associated with the change. If the body is believed to belong to your god, then modification might be prohibited without permission from said deity; or the god might demand it.
Tattoos can be part of culture, whether that’s the placement or style. We’re talking less about individual expression here and groups denoting membership for their individuals. We can choose black or other colors, use symmetry or not. Women might have more feminine styles while men have bolder ones. A religious group can require them, as can social groups wanting a specific tattoo to show membership that must be condoned. In some cultures, it might only be criminals and gangs who wear them. Some tattoos, like henna, are not permanent. As for what’s shown, these are typically symbolic, even if only being lines of some kind.
Piercings are another body modification that can be more than an individual’s taste, but part of a culture. Many body parts are available, and invented species might have more, but ear and noses are the most widespread and ancient on Earth. Stretched ear lobes and lips are another form. The number, size, material, and style of piercings can all be expected and represent a value. They can be signs of nobility or wealth. Gay men used to wear only one earring to indicate their orientation. Beliefs and superstitions can result in them, too, such as an idea from the Middle Ages that a specific piercing improved long-distance sight, resulting in explorers having them. There can be practical ideas, such as sailors thinking a gold earring can pay for their burial if they wash up somewhere.
Branding isn’t typically accepted on Earth in modern times due partly to associations with slavery; it not only marks property but can humiliate if the brand is always visible. It can be done as punishment, such as slaves who’ve run away, or military people who committed an offense like desertion. Any crime that we feel others should be alerted to can result in one. We can choose that those with inclinations deemed offensive by the state can receive one. We have leeway to decide where it is done (very visible or under clothing), how large, and its design.
Implants take on special significance in SF, where technology can become part of the body. These can enhance abilities and senses or simply replace lost or damaged areas. There may be backlash against this or full acceptance by all of a culture or only parts of it. Tension is always desired in storytelling, so it may be best for some groups to oppose while others adopt. Questions of authenticity may arise, in the sense that someone is no longer who they were born as, if enough changes have been made. Are they still human? Do people feel augmented or like they’re losing themselves? What is the psychological and philosophical impact of too much change, and where they draw the line at “too much?” What value is being offended or championed by the changes? This will decide how culture views it.