Sigil surroundings are exactly what they sound like, they are shapes, designs, and/or glyphs you put around your sigil in order to enhance its magickal properties.
The circle is a powerful symbol used in all different types of magick. It embodies the metaphysical properties of protection, unity, cycles and focus. The magickal purpose of drawing a circle around a sigil is to go through the process of allowing the sigil a liminal space to work, to protect it from outside influences, and to allow it to more effectively build up power from the properties of the circle. Some practitioners will even use two circles around their sigils. Some of these practitioners will also fill the inter space between these two circles with other sigils, and/or a magickal alphabet of some kind to enhance the effectiveness of the sigil within.
The triangle represents the trinities, and embodies the metaphysical properties of ascension, manifestation, and duality. Sigils surrounded by a right side up triangle will be much more expansive, and active, and sigils surrounded by a right side down triangle will be receptive, and passive.
Square, And Rectangle:
The square embodies the metaphysical properties of structure, balance, and community. This information can also be applied to rectangles. Sigils surrounded with a square allows the sigil to be housed, and reserve its power. It is for sigils that are used in more of a balanced state where its energy will be used more like a battery, and called upon at specific times. Sigils that have a square around it can be called upon when it is needed, and because of this It is used more in passive activation methods.
Pentagram, And Pentacle:
The pentagram, and pentacle embodies the metaphysical properties of the elements, power, and protection. Certain practitioners would place their sigil in the very center of the pentagram, or the pentacle to allow them to be stronger, and to take on the metaphysical properties of the elements. Using the pentacle would also mean you would gain the magickal benefits of the circle also providing more properties to your sigil. Also the spots on the arms, legs , and head of the star could be used as a prime spot to place more sigils in order to enhance the sigil that is in the middle.